Hi,
Thanks for the message. Which version of Forest Pack and which renderer are you using? In recent releases, we no longer use the multi-sub material generated by Forest Pack for final renders. Instead the material on the source geometry is used directly and the multi-sub material only for viewport previews and in case of needing to support legacy versions of renderers.
In any case, the limit for multi-sub objects is 999 materials. This is a limitation of Max that I am afraid we cannot circumvent.
I hope that helps,
Paul