I just picked up a couple of those along with a cap nailer. Haven’t used the cap staplers yet, but as far as he nailer goes, it’s incredible. I can only imagine the same for the staplers.
As for your question regarding capping underlayment, that depends. If you’re going right over it with shingles, you just need enough caps to hold the underlayment in place while you work your way up the roof with the shingles.
If being left over night? Every 6"-8" at the seams and at roughly every 12"-16" in the field.
Matter of fact, if you’re shingling over your work as you work, you don’t even need a cap stapler/nailer…a hammer tacker is fine (stapler) since you’re going to go right over it right away.
Nails for the shingles? Depending on where you live anywhere from 4 minimum to 6.
Here’s a trick; the shingles come with instructions on the package.