Excuse for my English. Could not find the requirement to models by quantity of triangles. For example trees. What optimum quantity of triangles on model for the engine? Thanks.

you need to test it manually

What the most optimal is? 0 triangles.

So it's simple, try to use as few as possible. If it looks good, and you can place quite some trees that doesn't affect your fps too much, you should be ok.

This also depends on your target audience, low end machines, mid end, or high end (can target all 3 by using LOD ofcourse).