The short version is that filling up every wall with foam will result in terrible sounding reflections. You have to spread out the foam, think of creating a checkered pattern where if you put up a square foam on one wall, on the wall directly opposite that piece of foam do not place any foam.
Bass traps are meant to go in corners, it’s fine to put bass traps in every corner where two walls meet in a room.
You should take measurements of the room you wish to treat, before shopping for full sets of foam. This way you’ll know exactly how many pieces of foam you’ll use, rather than finding yourself with extra foam and nowhere to put it.
The most important thing about putting foam up in a room: Do not use glue. Foam is porous so the glue that’s sold with foam will not be absorbed and the foam will fall down off of the wall anyway, the paint on the wall will peel off with it. The best way to put up foam is to use good old T-pins. Minimal damage to both the foam and the wall, T-pins are easy to take out when it’s time to move the foam.
This topic is easily googled, here are some essential links for your homework:
How to Control Sound Reflections in a Voice Over Recording Studio
Recording vocals – a good setup, basic acoustics and accessories
The Secrets of Bass Trap Placement
Home Studio Foam for Your Home Recording Studio
Here’s a video showing the effects of foam placement on sound:
Fryda Wolff 