As long as you are only copying the channels, or talkgroups from a ‘group’ into another, already existing group, that does not bump your group total, for the target system, up.
If importing these, you would manually select only up to 20 of the groups in a system for import.
You would select and import those. Once you complete the import, start another import session and get the system (site) details, then select up to the “next 20” groups into it. Once that import session is done, then you can drag and drop selected channels from a group in the “second” system into a group in the initial system created.
Remember that after your “first” import session, you can combine (thin out that number) of groups in that first import session before having to do a second import.
It’s not as complicated as it sounds. I’m using my phone to answer, so I can’t give you screenshots right now. About to call it a night; have an early appointment tomorrow. If still somewhat confused, I could probably create a few screenshots showing this, but that would be, at least, sometime tomorrow afternoon or evening.