Creating the buttons on a calculator using a for-loop
Basics
Classes in Java naming conventions should start with an upper-case letter: miniräknare should be Miniräknare.
Variables in Java naming conventions should start with a lower-case letter: Inmatning should be inmatning.
Refactoring
There are a few things in your code that can be refactored to improve the readability. Functions are a part of that. Consider a function that adds a digits to the display:
private void addDigit(String digit) {
    textField.setText(textField.getText() + digit);
}

That function removes the display logic from the action listener, and puts it in the miniräknare class itself.
Another trick in Java is that interfaces with a single method, like an ActionListener are implicitly a functional interface. This allows instances of them to be declared in functional shorthand....
nummerButton[i].addActionListener(e -&gt; addDigit(((JButton)e.getSource()).getText())

One more thing is that the font can be extracted to a constant...
private static final Font keyFont = new Font(&quot;Tahoma&quot;, Font.BOLD, 20);

Code order
I know this one is pedantic, but when you have &quot;setup code&quot; and &quot;use code&quot;, you should put all the setup code first before you use the instance you have set up.
In your case, you have the line of code:

frame.getContentPane().add(nummerButton[i]);


before you add the ActionListener to nummerButton[i].
Also, I would avoid the nummerButton array entirely until the button is complete....
Result
    int kolumn =((i-1)%3);                      
    int horisont=((i-1)/3);             
    JButton key = new JButton(String.valueOf(i));     
    key.setBounds((10+(kolumn*60)),(220-(horisont*60)),50,50);   
    key.setFont(keyFont); 
    key.addActionListener(e -&gt; addDigit(((JButton)e.getSource()).getText());

    nummerButton[i] = key;
    frame.getContentPane().add(key);        

If you want, you can put all the JButton creation code in a new function too, but that may be too much refactoring in your case depending on how much other code is in your method....
private JButton createKey(int value) {
    int kolumn =((value-1)%3);                      
    int horisont=((value-1)/3);             
    JButton key = new JButton(String.valueOf(value));     
    key.setBounds((10+(kolumn*60)),(220-(horisont*60)),50,50);   
    key.setFont(keyFont); 
    key.addActionListener(e -&gt; addDigit(((JButton)e.getSource()).getText());
    return key;
}

and then your calling code becomes:
    nummerButton[i] = createKey(i);
    frame.getContentPane().add(nummerButton[i]);