Redoing the ductwork for my (old) house
Hard to give any specific ideas about duct design without a site visit, or lots of pictures.
It's always good to have returns to each room (except bathroom, kitchen, laundry, etc), but that can ratchet up the cost of ductwork. Two returns upstairs may be all you need - or you might need more, or perhaps only 1. Impossible to say without some knowledge of your layout.
As to returns in the unfinished basement: Unless you're going to condition that space, then you should not return air from that space. Adding a couple of supply registers won't do you much good either. You should either condition the space or not, but don't do it halfway. Also note that if you plan on doing this, then almost certainly your system would be undersized, so take that into consideration.