Excellent news! The number of supported federation service (FS) servers in a farm with a Windows Internal Database (WID) backend has increased from 5/10 to 30:
A WID farm has a limit of 30 federation servers if you have 100 or fewer relying party trusts. If you have more than 100 relying party trusts, a WID farm has a limit of 5 federation servers.
From the AD FS design documentation: Federation Server Farm Using WID.
For MANY customers and implementors this means no SQL, which for AD FS is nothing but a good thing!