I rewired the car, will have to double check what amp that circut has. Will a stock replacement handle the 20amp? Might need to bypass the switch and just wire the running lights and dash lights to their own switches. Those are the only two circuits that didn't come on with the factory switch I have.
The protection (fuse, circuit breaker, etc) is sized on the weakest link in the circuit. In theory that's the 18 gage wires to the headlights. In reality its often the terminals and terminal crimps in the connectors. Especially when oxidized.
The original headlights drew less than most replacements. If you are running 6012 lamps, they'll draw less current in both hi and low. But very few folks still have or install 6102s. Mostly guys going to shows who want the most original parts.
A 20 amp circuit breaker will not hurt regular operation even with a typical replacement lamp, like 6014 or H6014. The problem with a 20 amp CB is if there is a short, it may not open before the wire or a connector melts.