Basic SSLVPN is free and unlicensed. You limits are the physical resources and size of appliance. If you need host protection and what to push polices fo security at the client the forticlient has to be licensed and that cost money.
...and the "0/10" means that you haven't registered any FortiClient of 10 free licenses per Fortigate. As emnoc has posted, you only need to register the FC if you want to manage them centrally on the FGT: push configs, push signatures, check connection status etc. You can use as many unregistered FCs as you like.
