It very much depends on the habitat or ecosystem. For a scene like this, from my head, it's oak (3 types), birch (3 types), hazel, hawthorn and linden for the higher areas, and a 2-3 species of willows and 2 or so black alder (I often use a patch of 5 alder trees as one object if they are to be in a wider area, and also have some patches of fallen alders) for the lower areas. Then I have some indistinct shrub (either single or as a small group as one object), 3 types of grasses or so, some herbs for the front, some reeds and swamplike vegetation for the wetter areas. But what you see here is only a top small higher area.
On average I'd say 10-20 objects.
Some 4K RGB (in TG to be split up) image maps as masks, but also in combination with altitudes, and occasionally an extra simple shape or painted area in a later stadium when something doesn't quite work with existing masks. I use a lot of multiply, add, etc. blue nodes for these combinations.