It actually depends on the site from which you access your private messages. Stardock has 12 different "forum sites" (and 2 associated blog sites but that's a different story).

http://forums.demigodthegame.com/
https://forums.elementalgame.com/
https://forums.galciv2.com/
http://forums.impulsedriven.com/
https://forums.joeuser.com/
https://forums.politicalmachine.com/
https://forums.sinsofasolarempire.com/
https://forums.stardock.com/
http://tgnforums.stardock.com/
http://forums.vistathemes.com/
https://forums.wincustomize.com/
http://forums.xpthemes.com/

Demigod, Elemental, Impulse, Political Machine, Sins, Stardock, Vista Themes and XP Themes all list the PM limit as 50.

Galciv2, Joe User, TGN and Win Customize list the PM limit as 100.

I have had occasions where I’ve had over 50 messages and it has worked just fine but I’ve never tested it on a site with a 50 message limit, so I don’t know what will happen. Since I discovered the site variation in PM limits I’ve just made it a habit to keep my mailbox comfortably below the 50 PM limit “just in case”.

I took the liberty of posting this to the "Forum Issues" forum which unfortunately is not accessible from Demigod or Elemental but is accessible (/forum/503) from pretty much every other SD forum site. Here's the link to the report for your reference, https://forums.galciv2.com/342335.

[edit]

As a note most of SD's forum sites have a dedicated "site issues" forum that exists under the Misc or Off-Topic forums. Elemental seems to be an exception but Demigod does have such a forum (http://forums.demigodthegame.com/forum/505) although it's not listed directly in the top level category list. Most other sites also have access to the Forum Issues forum for problems that are universal across all sites (just append /forum/503 to any of the url's that I listed in my first reply) and again that usually exists under the Misc or Off-Topic forums.

[/edit]

Sure, no probs.

Actually both elemental and demigod should have access to the "Forum Issues" forum.

I put in a request to fix that here, https://forums.galciv2.com/342503.

It can be confusing to get a new game and come to the site dedicated to it only to find yourself thrust into a web of 12 different sites with widely different interests, all of which partially share some of the same forums. Often it's hard to figure out where a thread is coming from.

Anyway the forum issues forum is to report problems with forum software that is common and shared among all 12 (and counting) sites whereas the "site issues" forums are dedicated to problems specific to any one site.