Hi...
It is not an exact number. It is what you are measuring..
First: make sure you start to have cash flow and REAL customers
Second: is this cash flow sustainable??
Third: when do you expect to reach a breakeven point? Is your MVP enough? Or future investment is needed? When do you think , based on current customers, you will be able to break even again the new investment?
Fourth: most important thing is Growth.. How do you plan to grow?
By more sales? Or improving efficiency? And is the growth sustainable ?
Is it a constant growth ? Or variable?

Also, reviews from customers and customer satisfaction for your MVP (paying customers) will be your 100% real guide, coz reviews most of the time give you false positives..

Good Luck!