I doubt that many of you will ever run into this limit, but for those who need to manage large data sets: the Group Tree in versions 9 through 12 will simply stop at group number 65,500. The groups beyond that point will still appear in the report (both preview and print) but will not show up in the group tree.
The good news is that this limit is applied per node. So, if you have a single group level with 90K groups you will only see 2/3 of them in the group tree. But if you add a primary group that splits the list in half, then each node of the primary group can show up to 65K groups.