Our club used to shoot airgun benchrest matches at 50 yards for group for many years. After months of trials, beginning with ten shot groups, then down to five shot groups, we finally settled on three shot groups, BUT, we fired THIRTY three shot groups per match, then measured all the groups ctc, with the winner having the best aggregate.
The REASON for three shot groups was the difficulty in easily determining how many shots were really in a group when measuring them. See, with tight clusters, its not easy to tell. Yes, we DID use backers for awhile, and they do work, but its a pain to use em, and keep them changed, so we eventually found its darned rare that a three shot group is confused with one or two shots.
Typical decent sporting airguns deliver around .75"-1.00" thirty group (90 shot) aggregates with decent ammo off the bench. Good tuned sporting, or FT competition type airguns give .5-.75 aggregates. Really well sorted dedicated benchrest rigs give .35"-.5" aggregates. We found .22 cal rifles shooting around 30fpe generally the best, with the record setting guns here in Temecula over a several year period being USFT rifles using sorted .22 JSB pellets and Benchmark 2 groove barrels. The record aggregate was a bit under 0.34" for the thirty three shot groups, with many groups under 1/16" ctc.
When we began to shoot the International target (one shot per bull) we stopped group shooting competition, and, I suspect, have likely lost some of the knack of it. It took better eyes and skill to accurately measure all those groups than it does the 25 meter International cards, so we haven't gone back to the "old game".
Group shooting in competition is different than "Centercut" cites, since one normally "chases shots" to keep the group tight. Our game required at least one shot to touch the black bull, but as with most group shooters, we normally offset the scope zero to avoid obliterating the actual aimpoint, which was typically at six or nine o'clock on the black. Ideally, one or two shots were in the black, but one or more could be in white without penalty. Failure to "anchor" a group to a bull resulted in a 1" penalty added to that group.
In general, if you shoot enough groups so the AVERAGE is good, its impressive, but one TINY group is really not too much rarer than the occasional HUGE group, if you do your part. To prove out a rig, I think ten five shot groups is enough to show potential.