With no stated number, a client naturally asks for adjustments until they're happy, which can mean one round trip or ten. It's not bad faith, it's the absence of a boundary that keeps pushing them to ask for a little more.
A number in the quote, not at delivery
The most common benchmark among photographers: one to two revision rounds included, on a set number of files agreed in advance. The exact number matters less than the fact that it's written down and known to the client before the job starts.
Stated in the quote, it's never felt as a constraint; stated at delivery, it always looks like a last-minute refusal.
Separate a revision from a change of creative direction
A revision corrects (brightness, cropping, a blemish); a change of creative direction asks for different work (a new color treatment, a new selection of photos). Frame the included rounds around revisions, not around fundamental changes, which belong to a new job.
Centralize requests, don't let them scatter
The real cost of revision rounds isn't the number, it's the scattering: a voice note, an email, an Instagram comment, each with a different request. Centralize revision requests in the same place as the delivery, specific and numbered.
xFer lets you receive the revision request right on the delivery page: a round is counted and closed cleanly, instead of dissolving into ten separate conversations.
In short
State the number of rounds in the quote, separate a revision from a fundamental change, and centralize requests in one place. The boundary protects your time without ever frustrating a client who knew what to expect.