It's always just been me. This is a lifestyle business for me, and I don't really feel time-constrained at all. The variety of skills you have to develop to run a software business alone is what makes it interesting to me; I never get bored with the work since there are so many roles I can choose to take on each day. There's always something new to learn on the horizon, and great motivation to do the learning (like ack! you're about to have more traffic than one server can handle, better learn how to handle that!).

The only downside to working alone is being on-call 24/7/365 for the servers. I have a good alert system that will wake me up if anything goes wrong overnight, though most components of the stack can fail without taking down a site. Lots of load balancing and warm failovers at a second host just in case.