Is there a limit to how many credentials SSMS can remember?
It is still a bug (sql server 2008 - 2014 (version 12.0.2000.8 still affected)) as highlighted below :

SQL Server 2012 Management Studio spuriously forgets password, despite being set to remember it.

You can vote up the connect item to get it HOPEFULLY fixed if you are experiencing the bug as well.

%UserProfile%\AppData\Roaming\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin


might have a limit but I don't know of any kind of hard limit.

BTW, how big is your sqlstudio.bin file now ?