I don't think there is a "magic number" although I do think having 10 or 20 people giving you your credit card or cash is enough to get started. (e.g. Groupon sold its first deal—two pizzas for the price of one—in October 2008. Twenty Chicagoans bought in and trekked to the Motel Bar, located . . . on the first floor of Groupon's building.)
Although I will say that you might be looking more for a number like Amy Hoy's 30x500 - as 500 people at $30 = $15k MRR
Just a thought.
