You can start making moves at even 100, so long as the people on the list are into you and what you're doing (rather than fly-by-night joiners, or people bribed into joining your list).
But 500 is great (with that same kind of audience), and by the time you get up to a few thousand really engaged, interested readers, you're set. You may not make a livable wage right away, but you'll be making SOMETHING, and from there all you have to do is keep doing good work and publishing it (and as you build a back catalog, each new release can lead to a big boost in sales of your past work, as well).
But 500 is great (with that same kind of audience), and by the time you get up to a few thousand really engaged, interested readers, you're set. You may not make a livable wage right away, but you'll be making SOMETHING, and from there all you have to do is keep doing good work and publishing it (and as you build a back catalog, each new release can lead to a big boost in sales of your past work, as well).