It depends on many factors - hard to put it in universally applicable numbers.
That being said, having even a single paying customer, unless it's a close relative/friend, is an achievement in itself. Still, personally, I'd feel better having at least three paying customers. One might be a fluke, two - a coincidence, but three and more - that's either a beginning of a trend or a lucky strike (both are good :-)
Unless, of course, your product is Enterprise Software, in which case having even a single paying customer is one hell of a start - shuts up all the "unbelievers" like that.
Of course, 3 or 10 or 50 is just a start. The real test of a paid business model is to estimate the CAC (customer acquisition cost) and the average LTV (life-time value) of these customers. LTV should be at least 3 times the CAC, or you won't be profitable. It's tricky, since there are many hidden costs that are hard to see, but this calculation is imperative.