I purchase a ringtone from iTunes. Would like to know if it’s a one time charges or monthly ?

Like any other music purchase from iTunes its a one time payment only.

i have never seen a subscription for ring tones so would be a 1 time charge. If you have a computer you should also be able to download it, just in case some day it is no longer in itunes store

FYI

View, change, or cancel your subscriptions - Apple Support