Enter the KCBM comp, we give out 50s like candy
(tomsawyer got one of them).
Seriously though, I have given as high as 46 in about 500? scoresheets filled out. My highest (consensus) score received was a 45 I believe in maybe 125 beers entered.
Scores seem to vary a bit in generosity from local group to local group but I would say average is around 30-32 and a 40 is the top 10th percentile if not a bit more rare. 50s should be extremely rare, like once in a lifetime events for a judge or an entrant (if that).
The general point being that the whole 50 point scale is used*. This isn't Beer Advocate where anything that has seen a barrel is guaranteed a high score. Nobody should feel bad about a score in the low to mid 30s or even one in the 20s (good brewers get them all the time). When you talk about a score like 48-50 you should consider what that (hopefully means), the judge fully expects this to be one of the best small handful out of hundreds or thousands of examples tried in that style.
* This depends on your philosophy and the comp rules but typically very bad beer scores are truncated at a courtesy score of 14 or something like that.