As to how many brushes, that's a relative thing. You could make a darn fine map using a single symbol for the mountains, but if you're getting into variation, I find that having 30 mountains feels a tad limiting to me at times, 45 to 60 would make me very happy

Trees don't need as much variation, I find, I'm happy with 20 or less. They're usually at a size where you can only make them look so different after so many tries.