If you want to use more than 512 addresses, you should use an external DHCP server.
If you want the full use of all your addresses without having to carve out duplicate addresses on the opposite controller, you should use an external dhcp server.
If you want your clients to be able to use the same addresses upon failover, you should use an external DHCP server.
The internal DHCP server has a limit of 512 addresses
It is not synched between controllers, so you would have to cave out addresses that are served up on each controller and your capacity gets halved upon failover.
Your best bet is to prepare to use an external dhcp server.
The internal dhcp server on the controller is only intended to be used for guest traffic.