My profile has over 150 different collections (I use them to represent my archived CDs, one collection per archive box).
I can only see the first 25 on my collections page, and there is no button on the web page to transition to the next set of 25.
Interestingly, I can manually append ?page=2 to the collections URL and get the next page.
Using Chrome and IE11 under Windows 7.

https://musicbrainz.org/user/debris77/collections
https://musicbrainz.org/user/debris77/collections?page=2