Tuc
I am not sure the exact reason why they(GARMIN) limit the number of maps able to be loaded onto a card, but I have several ideas.
The first is the ability of the software to handle such a large array in an efficient manner, meaning that the programming would be exponentially slower above a certain point when trying to read, start-up or calculate things. The ability and power of the GPS processor would impose a limit at some point also.
Another is a matter of transfer speed, especially when using the GPS as the loader/reader of data through USB connections; which are still rather slow compared to direct transfer from HD to memory card.
Here area couple of links with tons of extra info, and even a way to get around the limits,possibly.
http://freegeographytools.com/2007/g...sd-card-tricks
http://gpsinformation.net/