What amp breaker is used on a 18.5 minimum amp ac unit with 8 ga supply line.

I think with an 8 ga supply line the max breaker you could put is 40 amps. That doesn't mean you need 40 amps. You may be able to get by with a smaller breaker, depending what the maximum current draw is. It's 18.5 amps minimum, but does the AC unit specify a maximum current?