What can I do with a lot of styrofoam squares?

You didn't mention the size of the squares but here are some ideas:

https://www.hawthorneandmain.com/21-amazing-diy-styrofoam-projects/

https://www.parents.com/fun/arts-crafts/kid/styrofoam-kids-crafts/

Yes... you can decoupage them... wall paper, photos, wrapping paper, fabric... Pinterest.com may have some ideas for you

I use styrofoam squares to put in the bottom of tall outdoor pots that I don’t want to fill all the way up with dirt (makes them too heavy to move), then I put 6mil plastic from middle to top, put some dirt in, then put my plants in. You can also cover them with fabric, I use them when I want to paint screw heads, just stick them all the way down and paint away.

How many is ' a lot' ?

If they are not the kind of styrofoam that breaks apart quickly and is messy, then donate them to a local kindergarten or elementary school. They make a great resource for play or building 3D towers, etc.

You could make beautiful artwork out of them using modge podge to decoupage pictures, leaves or basically anything onto them. And they'll be light enough to hang up too.

I'm sure you'll do a stunning job and you can test the modge on a small piece of the foam first. It should be okay though.