Constructive possession isn't really BS...but you are unlikely to have an issue unless you are dumb as a stump. Technically, if you do NOT have a SBR lower (meaning stamp-in-hand) or a pistol lower, you can't have short uppers. So if you decide to get ahead of things, don't take the short upper, put it on your plain jane AR rifle lower and take it to the very busy range to show it off. Could go wrong. Otherwise, no sweatie-da, GI.
But the answer to your original question is as was already said is no limit.