So, how many trees you need to chop down to find a seed?
I don't know if it is because a have other mods, but i can't find any seed at all

You should be getting seeds from every tree you chop down. It's most likely a conflict with other mods.

One conflict that I know of (sorta) is the Aussybrush mod (Fire-resistant brush mod). OTOH, while using the Exploration Vehicle Mod, I noticed that was collecting seeds in my inventory, and scattering Aussybrush saplings in the wake of the modded vehicle.

Go figure... :-p