I'm told that it's best to have a lot of returns in your system .
If I add some what will that do?

It will aid in moving the warm air (in the case of A/C) back to the evaporator coil to be cooled and then supplied to the home. A lot of returns is kind of an overstatement. If located correctly one is just fine and of course the size is compatible to they system. Adding another in a 'hot spot' can help get air moving into and out of that spot.

Hope that helps. Location is more important than number but that is just me.

I agree thermal, size and location is most important.