You can use unlimited free domains in FunnelFlux Pro.
The catch... these domains must be managed via Cloudflare and have the "orange-cloud" mode enabled so that Cloudflare proxies the traffic and provides the SSL.
The main complication with custom domains is SSL provisioning, and since Cloudflare handles this side of things, it makes it easy for us to let you use as many domains as you want.
Simply add them in the FunnelFlux control panel and set them up on Cloudflare in the same way as usual, based on this guide.
We will soon be migrating to a new domain approach that does not require you to use Cloudflare, though it is highly recommended.