Hi, i want to ask a single question. Will it be too high poly for a tree with 6396 polygon? See the images below. It is the tree model i made using 3ds max. I planned to include the tree into my game project.

It depends how much else will be in the scene. Although 6000 polys for a tree does seem a bit high when a lot of games use that amount for humans etc.

I'm not really sure about the polygon count but the tree itself looks really good. :)

How many of these trees are going to be onscreen at once?

hmm not sure yet...
if the map is a forest... should be many
anyone here knows normally a tree model has how many polygon?

And... below is the test terrain rendered in 3ds max:

Simply, yes, its too many. You want to aim for nosmore than 1000 polies/tree (for comparison, a character is usually between 3000-4000). The best trees I've seen so far have been in Elder Scrolls IV: Oblivion. Each cluster of leaves is its own billboard, and it looks fantastic. Look at how they do it.
If you still want to use that model, get rid of all of your smaller branches, and reduce the amount of sides on your canopies and the tree itself. A wireframe shot would help, but you can cut your polies in half by using half the amount of sides.

Elder Scrolls IV: Oblivion << the tree models are really cool
how they make those fantastic models?

might be, engine effects/lighting.

They use SpeedTree technology.

You can render the tree from different angles, then paste the renders as textures onto a simpler model to make it more workable as a game object.