This is according to Elaine (or perhaps Julia Louis-Dreyfus). I saw it on some special on E! while channel-surfing (and simultaneously talking on the phone), c. August 1996. Don't know exactly what the special was about. (I didn't even have the sound on, but caught it on the closed-captioning.)
Apparently (according to alert reader Tom Matches) this fact was also mentioned by Elaine to Jerry on an episode of "Seinfeld". I'll have to look into that.
Alert reader Paul McAllister reconfirms this factoid, having caught this episode on a rerun. He further reports that Elaine and Jerry proceeded to sleep together later in the episode, making for more than 37 total. Oh well.