Biggest mistake I ever made was not focusing on one to exit. I thought once I bootstrapped the first business to a mid-six figure subscription level, that this was easy and I should fund all my other ideas with the proceeds...
Meanwhile I used close to a million dollars over the next few years developing a whole lot of stuff that is unlikely to ever see the light of day. Plus, the lack of attention to the primary money earner caused it to suffer.
My advice: Choose one that you are most interested in. No side projects that have any possibility of making money. My new side projects are all hardware related or blue skies science to reduce the temptation.