Unless you take some other actions, every time you send a map to the Montana with Basecamp, it will be placed in a file named gmapsupp.img. Because of this, it will erase any map that you previously sent to your Montana. So there is only going to be one map on the Montana, no matter how many different ones you send.
The way around this is to manually change the filename from gmapsupp.img to something else, like topo.img. Now you can send another map which will again be named gmapsupp.img. Change that name to something else again and repeat for every map that you want to send.
When you go to the map setup menu, you will see a list of all these files and you can enable/disable them individually there.