There are 2 different limits, the amount of different mapsets (20 on the Oregon 600), and a map-segment limit of ~4000 for all loaded maps together.
The GB raster maps don't have much map-segments in total so that's no problem.
Your Montana can make use of 32GB cards in principle, it depends on the type of maps if you can fully load it with maps. My experience is that you'll reach the 20 map limit or the 4000 tile limit around 12GB so 16GB cards are the best choice. If you have a built in camera it's usefull to have larger cards offcourse.