One per round sounds about right for my usual game, most of them lost balls in water hazards but also a few that get nicked up (cart path, tree, etc.) and I throw them away after the round.
That said, when I play courses at the beach or somewhere with lateral hazards everywhere or tough courses with long rough I may end up going through the better part of a dozen balls over three or four rounds. I hate lost-ball courses like that although I prefer the water hazard ones to the ones with tall, unfindable-ball rough. I'm not patient enough to spend five minutes looking in the tall grass for my own ball, much less the ones belonging to the rest of the foursome.
So all told, call it 8-10 dozen balls per year. That's for 75+ rounds at home and maybe 25 more on the road.