The 'Undo Limit' is what setts how many undos you can do.
However no matter what you set for that, there are certain command in Archicad that will delete or clear the undo queue.
I found this in the Reference Guide.
Warning: The undo queue is cleared entirely whenever you do the following operations:
– Delete or add a story, or change story height
– Update a Hotlinked module
– Send/Receive commands in Teamwork