DHCP over a router and Scope Naming

[DarkOlive871]

I'm working with a FortiGate running version 7.4.9. The current setup has a single subnet between the FortiGate and our on-prem router. However, behind that router, there are many VLANs and subnets on the LAN. I’d like to move DHCP services for those internal VLANs to the FortiGate, but the FortiGate doesn’t currently have Vlan interfaces or routes for those downstream subnets. 

What’s the best way to configure DHCP on the FortiGate in this scenario?

Or is there a way for the FortiGate to serve DHCP to those VLANs without having direct interfaces in each subnet?

I have the DHCP Helper disabled but already setup. How do you Name the DHCP Vlans? 

I also didn't see a way to name the scopes? 

Thanks in advance for any guidance!
FortiGate  

[AEK]

36 will not be possible with this small FGT. If you have a separate DHCP server (like your AD) it would be better.

[DarkOlive871]

Anyway, to change the limit? 

[AEK]

As per my knowledge it is not possible to change.