I am currently insulating my basement. I left the studs far enough away from the concrete wall so I can put r20 in without it compressing. My main question is there is enough space between the studs for a piece of r12 insulation if I cut it, should I do this to reduce thermal bridging or is it useless?
You could consider getting spray foam insulation as it will fit in readily between and around the studs. Usually this is something you would have a pro do, etc.
Assuming your basement is dry, no moisture problems etc. and also assuming you are talking about blanket fiberglass insulation, here is my answer. The amount of work required to put one and a half inch wide slivers of insulation behind each stud is a total waste of time. "R" value on vertical walls is actually not as important as trapping air and preventing drafts. Don't get me wrong, the more "R" the better, but the majority of heat loss is up and compromised by cold air drafts. Just install insulation between studs, you will be fine. Remember, in your case, the vapor barrier goes between the concrete and insulation, not under the drywall on top of the studs.