the export need to be maximum 32.768 triangles meaning about 2730 items (12tris each)
(depends the quality of items i'm going to distribute, some lower poly are 8tris each so 4096 items )
How many generated i can't say, depends really
but assuming i have some already done forests that i'm copy pasting to a new area, similar size, it can sometimes go to triple the amount or more, so say 10.000 items instead of the 2730 limit.
So we're not talking millions anyways