I dont have a set # of shots I practice with. I limit myself to 4 arrows per string but sometimes I'll shoot those 4 arrows for hours & sometimes just one go round. Its the first one that matters the most but I just like shooting. Guns, Bows, I just like it. Shoot the bow more simply because I cant shoot my guns in my yard.
My groups open up after awhile & I'll take a break if I feel I'm not up to my own expectations. I shoot 68# on my primary bow, an older Martin Cougar speed flyte. I dont do any exercises specifically for bow shooting but do work out with weights pretty regular mostly bench pressing & I'm sure that helps a bunch. 6" at 20 yards while not bad isn't good enuff I'd go hunting IMO. That 6" can turn into 12 pretty quick when your freezing or after sitting for hours & then needing to make one shot count from a less than optimal position.
I practice from my shed roof, sitting down, from tree stands, from my blinds & every which way. My neighbors think I'm a whacko but I generally kill a deer if I shoot at it & thats what counts.