Not that this proves nothing with accuracy but, according to Tom Lord Discography, there are quite a few pianists with many more sessions than Bill Evans:
-Duke Ellington 1109
-Hank Jones 933
-Count Basie 609
-Teddy Wilson 557
-Fletcher Henderson 514
-Jess Stacy 489
-Herbie Hancock 464
-Stan Kenton 445
-Jimmy Rowles 444
-Oscar Peterson 412
-27 other pianists, from 379 to 242 (among them "rarities" such as Irving Brodsky, Arthur Schutt. Chummy McGregor, Bengt Hallberg or Paul Smith)
...
-Bill Evans 240
With the old version (4.4), I compiled a list in Excel for all the musicians included in the index with more than 25 sessions, that I can order alphabetically, by number of sessions or by recording years (yes, I do stupid things sometimes, I know)