It would be interesting to see the breakdown for software/internet companies founded in the last 40 years. That's the data that matters for most people here.
I personally would be most interested in web companies founded in the last 10 years; I think that conditions have changed a lot recently. The Alexa top 100 would be a good-enough proxy. I pledge One Upvote to whoever hunts that data down.
A software / internet company doesn't have to build factories, doesn't have to build hardware, doesn't have to coordinate nearly as complex a marketing campaign, doesn't have to raise as much money, and doesn't have to have as many engineering disciplines on staff.
Thus, where Apple neede hardware and software and marketing guys (really three founders, though Woz should count double)... plentyoffish.com only needs one guy. And when Apple was at the revenue level pof is at now, I think they probably had a thousand employees, probably at least hundreds.
so, the requirement for multiple founders (if it ever existed) is now less likely.
I know I'm not answering your question because its an academic question for me-- I've chosen my path already.
I think really, at the end of the day, number of founders has no factor on the success of startups, but founder fights are a significant factor in startup failure.
So, whether you are solo, or you have 5 other founders, you need harmony between you and your other founders and/or you and your other employees, to have success.
I would guess it takes roughly the same effort as ever. That is, the tools exist for a single person to be massively effective now (Skype, IM, eFax, EC2 8-core instances, S3, AdWords/AdSense, turn-key solutions for all sorts of services, more and better programming libraries and frameworks and documentation) but the sophistication required has increased with the learning curve.
You had to work all the time at your start-up back then and you still have to now. The gains in complexity have kept pace with the gains in efficiency in a sort of self-feeding co-evolution.
