I'd say you need 500 players, 200 of which are active in a given month. Although it really depends on so many factors that it's impossible to give an accurate answer.
In any case, you do need at least a year before you can count on your group of players becoming large enough to be making you some decent income. Especially because you won't be sending all players to the same book, you'll likely be sending players to at least a dozen of them, so it will be more difficult to reach the 500 mark at any individual book.
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