Limit to number of sensors attached to Arduino
There is no answer that can be given to you that is "correct". However, there are things for you to consider in your design process:


Can you fit all the code in flash, and the required variables in RAM? 
Does the chip have all the IO you need, and can you use the required internal / interfacing peripherals in the right combinations to satisfy your requirements
Do the devices fit within your power budget (battery capacity, required run-time, power regulation efficiency).


The Arduino is just a component. It belongs in a circuit, and that circuit is what determines everything else. The ATMega328P has certain power requirements, as do all the other components in your circuit. It is up to you to design the right power system to provide that power with the right efficiency and power saving options (switching off circuit segments when not needed to save power), etc.

For prototyping and experimenting the Arduino provides a rudimentary power supply which is wildly inefficient. Not recommended for any battery powered scenario or any high current usage.