Hi. It's always better to to attach Your max file for reference. We can understand the issue and reply much faster than having Your description alone.
We could see Your renderer/Max Version as well as all the parameters You use and could give exact steps how to fix it.
Display parameters are for rendering as well.
I would highly recommend You watching this tutorial
. It explains distribution settings in very high detail.
There is "Max Items" limit for both viewport and Rendering. You can set it to 0 to make it limitless.
If You are using vray + billboards then optimising materials has no effect(It is very fast step You do not kill it too soon.).
Please manually add forest Color map:
Hope this helps.