What I think is that the total maps you do help to measure your experience like you said, but they don't mean everything. For example, you took 3 maps to get your rivers fine, others may take 6 maps, others may take one or two maps. Those who have the knowledge from school may be able to get the rivers fine in their first try. Experience can come from different sources of knowledge, you can learn from everything, all that matters is your learning skill. But time means experience in this case, because more time working on maps, more time of experience gain, the better you will get, you don't have to finish the maps, it can be just a practice (and Oh my!, I have done so much practice...) you can practice only mountain making, only river placing, and yet never get a map done.

To sum up things, I think that experience comes with unceasing practice, only big training can turn you into something really good.