In MapSource Canada it counts each 1:50000 Scale Topo Map as 1 Map, in the US I "assume" its 1 1:24,000 maps, but I don't have the US MapSource. These maps only cover a small area so 2000 maps would not allow for all of the continental US.
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Bruce Zawalsky
Chief Instructor
Boreal Wilderness Institute
boreal.net