Showing animated company KPI stats
Keep the global scope clean and data/code segregation

First I will say your code is good for a beginner but you are missing some concepts and techniques that will help you improve your code and improve the client experience. 

Some important concepts


Use requestAnimationFrame for animations, and know the difference between animation and one off content changes.
Data belongs in the page. Be it the text, images, or the instructions/values for the javascript, the place for data is on the page.
Keep the global namespace clean. With a little effort you can completely avoid adding any variables to the window namespace / global scope.
Don't put code in the HTML It is tempting to put code in the page to start an animation &lt;button onclick="startAnim()" value="start"&gt;. All good when it's a simple page but a source of headache, bugs, and unhappy clients when its get complicated
When parts of your code starts to look repetitive it's a good sign that there is a better way.
Remove event handlers when you are done. Event handlers use up resources. 
Always keep in mind your page shares the computer with dozens of others on the client's machine. Do your best to reduce the total load on their device.


The javascript    

Always start your code with "use strict" it helps you avoid some very common bugs and bad coding styles and runs your code significantly faster.

"use strict"; // is valid as the first line of a script tag or the first
              // line inside a function. Put anywhere else and it has no effect
window.addEventListener("load",function(){  // starting your code on the load 
                                           // event ensures that all the DOM 
                                           // tree is populated and ready


try to define your event handlers as named functions because you need a function referance to remove the event if you need to.

    function startButtonClick(){  // the start button event


We only want this to play once. So remove the event from the button. When events are called this references the element that triggered the event

        this.removeEventListener("click",startButtonClick); // remove event


The spread operator is a short cut way of turning a array like list into an array. Be wary as IE 11 is still popular and does not support many ES6 features. If you use things like the spread operator you should consider using babel.js so that your code can run on legacy browsers.

        var numberEl = [...document.getElementsByClassName('number')];   


The numbers moving up is an animation. It will be control by requestAnimationFrame that will call the animate function at 60fps if possible, but that is not a guarantee. To ensure you get consistent animation on all devices use the supplied time the requestAnimationFrame callback supplies to time your animation.

You were using a interval of 1ms.The rule of thumb is 60fps or 16ms between display frames, updating content any higher than that rate will simply not be seen and thus will just waste resources.

        var startTime;  // holds that animation start time
        function animateNumbers(time){ // time is the first argument of the 
                                       // requestAnimationFrame callback


This countSpeed controls how fast the numbers will increase per second. The animation will play at the same speed on all devices.

            const countSpeed = 120; // count speed in increments per second
            if(startTime === undefined){// if there is no start time then this must 
                                        // be first frame so make it the start time
                startTime = time;
            }
            var seconds = (time - startTime) / 1000; //seconds from animation start
            var count = countSpeed * seconds; // get the current count
            done = true; // flag to indicate the animation is complete
            numberEl.forEach(element =&gt; {  
                var max = Number(element.dataset.maxValue); // properties that you 
                                                // get from the  page are strings. 
                                                // If you know it's a  number let 
                                                // Javascript know


If the count is below max for any element flag the animation as not done.

                if(count &lt; max){
                    done = false;
                }


Try to avoid setting text content with innerHTML as it can cause all sorts of problems that are avoided by telling the DOM it is only text you are adding 

                element.textContent = Math.min(max,Math.floor(count));
            }
            if(!done){  // continue animation until done
                requestAnimationFrame(animateNumbers); 
            }
        }            


When ever you are animating anything on the page were you define every frame use requestAnimationFrame it ensures that the animation is in sync with the hardware and any other page rendering. It also prevents updated content from being presented to early or quicker than can actualy be seen. Smooth animation is not about speed, but about being in sync and limiting the amount of work the computer must do.

        requestAnimationFrame(animateNumbers); 
    }


Get the button element and add the event listener

    document.querySelector("#startButton").addEventListener("click", startButtonClick);
})


The HTML, data lives here

Placing the values for the counters inside the javascript code makes it hard to manage. The javascript is a constant, the page contains the data and is unique per view/client

The formal way to add data/attributes to elements is via the data- prefix though this is not enforced and any named value pair you add to an element can be accessed, data- ensures your data is safe from conflict or misinterpreted by the DOM.

data-max-value is not a valid javascript variable name so you can not easily acces the data using that name you need to use the javascript version created by the DOM. In HTML5 the DOM creates an attribute called dataset that you use to access the variable with. Replace data- with dataset. (that's a dot not a full stop)  then the max, then replacing any subsequent '-' by removing the - and capitalising the first letter following, thus data-max-value becomes dataset.maxValue see the code above as an example.

 &lt;p class = "number" data-max-value ="87"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Styles Created&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "286"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Awards Won&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "96"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Stores Worldwide&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "76"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Varities Desinged&lt;/h3&gt;


Avoid placing javascrpt inside the HTML content. It makes it much harder to manage your code. Use an unquie Id to identify elements for direct or queried access from within your javascript.

&lt;button id = "startButton" &gt;START&lt;/button&gt;


Put it all together



"use strict";
window.addEventListener("load",function(){
    function startButtonClick(){
        this.removeEventListener("click",startButtonClick); // remove event
        var numberEl = [...document.getElementsByClassName('number')];      
        var startTime; 
        function animateNumbers(time){ 
            const countSpeed = 120; 
            if(startTime === undefined){ 
                startTime = time;
            }
            var seconds = (time - startTime) / 1000; // get the seconds since the animation start
            var count = countSpeed * seconds; // get the current count
            var done = true; 
            numberEl.forEach(element =&gt; {  
                var max = Number(element.dataset.maxValue);
                if(count &lt; max) {
                    done = false;
                }
                element.textContent = Math.min(max,Math.floor(count));
            });
            if(!done){  
                requestAnimationFrame(animateNumbers); 
            }
        }            
        requestAnimationFrame(animateNumbers); 
    }
    document.querySelector("#startButton").addEventListener("click", startButtonClick);
});
button {
   position: absolute;
   top: 30px;
   left: auto;
   right: 30px;
   border: none;
   background-color: black;
   color: white;
   padding: 10px;
   cursor: pointer;
}
button:hover {
   background-color: crimson;
}
 &lt;p class = "number" data-max-value ="87"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Styles Created&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "286"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Awards Won&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "96"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Stores Worldwide&lt;/h3&gt;
 &lt;p class = "number" data-max-value = "76"&gt;
   0
 &lt;/p&gt;
 &lt;h3&gt;Varities Desinged&lt;/h3&gt;
&lt;button id = "startButton" &gt;START&lt;/button&gt;