Yes, and this is not limited to FileMaker 12. Earlier versions have the same limitation.
For More Information see: 51 Layout Pop-Up Menu Folder limit…
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks for the reply. Do you know if this is going to be changed?
I'm just a fellow user and so have no more access to any knowledge of FileMaker's future plans than you do. As you can see in the thread to which I provided a link, FileMaker is aware of the issue, but they never announce in advance what changes will be made for upcoming releases of their product.