Don't know if the OP is worried about the number of streams ONLY so he can write a paper on something.
But, I for one, rarely, get my water from streams.
Small seeps is my first choice.
Piped springs second
If and when I do have to get water from a stream, that's the only time I treat it (with chlorine), and like I said: that's rare.
Now. How many small seeps and piped springs are on the trail? thousands.