Record in player population would've been right after the game released in '98. I remember checking once or twice as a kid on my dad's dial-up and saw at least a multi-page list with plenty of populated servers.

I recall a big decrease in players happened after the official listserver went down and you had to manually add Burger/Becky's listserver address. Then eventually Nimrod continued this trend by setting up another unofficial server. I'm not sure when this all happened, I was rarely online back then. I do remember seeing usually no more than 1 or 2 servers online at a time. And most were locked clan matches, locked duels, or if you're lucky an open hotel.

Then eventually Nimrod contacted Rod & Cliff (?) from Epic and managed to arrange a redirect for the official listserver to his unofficial one. Basically, using vanilla JJ2 nowadays will still try to connect list.jazzjackrabbit.com which will then be redirected to Digiex. This relatively simple change made JJ2 way more active and resulted in more newbies discovering online play.