I found 100-200 clean watts on AM is about all I needed for mobile. Let me talk to anyone I could hear and for the most part let people who could hear me, talk back.
I ran as much as a kW for a while just to play with it, people could hear me waaaay before I could hear any of them, which was just sort of dumb.
100-200 makes CB during peak skip useful for local comms during daylight, otherwise you won't get over the noise level.