SSL users depends on FGT model and the total traffic + anything else your doing ( IPS, filter, UTM,etc....). Outside of that no hard-limits exists. If your seeing issues now, than that's a clue that it's possible your model is undersized.
Your option would be to upgrade the unit or add a dedicate unit as a SSLVPN gateway.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.