“You can get a decent sense of this by calculating your total addressable audience, and then using industry benchmarks for membership conversion to come up with a reasonable target.”
This was written for news organizations, but it provides a strong framework for how to estimate the potential of any membership or subscription product you offer.
The Membership Puzzle Project explains how to estimate how much you’ll earn from a membership program including how to calculate your total addressable audience, estimate monthly site traffic, and set targets to convert free subscribers into paid members.
Btw, I found this link in the Opt In Weekly newsletter.
Related: Four ways to grow your membership after it plateaus.