It appears to be 20 per page - actually no that hard when there's a lot of users in the PM conversation either - can confirm this now, it consistently links you to the 1st page of the conversation regardless of how many there are. However it does still provide the links to the other pages etc so it's mainly just a minor annoyance, it could certainly irritate those that use the PM conversation functionality extensively though.
There's another page bug with PMs - namely in the inbox (and probably outbox too) with more than 50 PM conversations there's no button to take you to the 2nd (etc) page, so you can only see the 50 most recent conversations without manually adding ?page=2 to the URL. Probably minor as well in the end - 50 conversations is quite a lot, especially if you delete PMs you no longer have any use for, some of us are pretty lazy when it comes to that though. My PMs currently date back to early 2008 (I could've sworn I had older ones than that, maybe I deleted them though) and that only amounts to 66 conversations atm (the 1st page alone dates back to 2009), though I haven't exactly been a prolific user of the PM functionality in the past few years.