I think everyone ought to learn standards in 12 keys, but realistically, how many can do them in more than 6 or 7? Or even have to? 12 is theoretical as it includes C# (all sharps) and Cb major (all flats), which are just spellings of Db (5 flats) and B major (5 sharps). So that leaves us at 10 keys, and we can take away 3 or 4 more that aren't used much without written charts.
Anyway, that is what it is. The question is what keys are particularly awkward on the C melody.
- I find Ab major is that way, because Ab = G#, and the G# key is its own nexus of awkwardness on any sax. (The only really good way to get from Ab to Bb is the Bb-bis key, which demands special attention in every scale where it's needed.) Standards called in 4 flats include I'm Confessin', I Can't Give You Anything But Love, Rose Room/In A Mellotone, and many more.
- I once got badly balled up on C mel in a jam session where Stompin' at the Savoy (Db major) was called. I'd been playing that tune since who laid the rails, but on Bb or Eb horns, where it's in Eb or F. But in the grand scheme of things, who cares? It was the 90s, it was NY, it was a here-n-gone bar full of vodka-soaked Russians, and I'm sure no one knew but my buddies in the band. Besides, for every 4 standards called in Ab, maybe one is in Db...Robbins' Nest, Stuffy...but if they call Woodyn You, duck out the back.