how many frets should a guitarist be able to span comfortably on a string?

Answer 1

Do you mean like holding a note then stretched to hit another note?

This depends on flexibility and dexterity but also your answer changes up the neck you can cover more frets down the neck less frets I'll give you examples.

From 1st fret to 6th fret pointer finger to pinky 5 frets.

Lets move up the neck now.
From 5th fret to 12th fret same fingers same distance more frets 7 this time.

Lets move up the neck one more time.
From 12th fret to 21th fret if I had 22 frets I could go to 22 actually. that is 9 frets now.

I will say this that is hard and a little awkward but I have seen Eric Johnson reach more that I can't reach.

Answer 2

It depends on the size of the guitarist's hands and how far that person can spread his or her fingers. Some people have more flexible hands than others so they can spread their fingers farther. That flexibility can also be achieved with practice. Both my daughters can play guitar while I can't. Because of that, they can both spread their fingers quite a bit wider than I can.