The answer varies from "1" to "as many as you want". The answer also depends on what you define as "startup". From the business's point of view the best thing

The answer varies from "1" to "as many as you want". The answer also depends on what you define as "startup". From the business's point of view the best thing is to have your whole time, attention, and commitment. Most businesses do not benefit from their founders' attention being divided (on the contrary, they suffer critically from this). But from a personal point of view, having your fingers in many pies is clearly beneficial, especially before you know which of your businesses will "stick". I think the answer to this is changing as the tools available become better at allowing us to be very effective with short bursts of work, but what's certainly correct is that you should not say no to opportunities - but be conscious of how much you can contribute to each and what it's costing you in terms of work not done on others.