I am building a model in the Materials Studio which includes many atoms, so I'd like to count the number of different atoms, is there any way to do that?
You have to set your interface to show the Properties Explorer, then select all the atoms of your system and look at the Properties Explorer.
In the examples below, there are a periodic system and a molecular one.
For the periodic system you will find the information in the Symmetry System filter under the Property Explorer.
In case of the molecular system, the information is under the Physical System filter.