Question: What is common among Hewlett Packard, Microsoft, Apple, Youtube and Google?
Answer: 2 founders.

William Hewlett and David Packard were research students of Professor Terman at Stanford University. Hewlett and Packard established “HP” at Packard’s garage in 1939, while United States were still in Great Depression. They tossed a coin to decide whether the company’s name should be “Hewlett-Packard” or “Packard-Hewlett”.

William “Bill” Gates and and Paul Allen established Microsoft in 1975.

Apple Inc., was founded by Steve Jobs and Steve Wozniac in 1976.
In recent years, we have Jerry Yang and David File of Yahoo!.

Then Google greatly influenced our life, thanks to Sergey Brin and Larry Page.

They acquired Youtube, which was founded by Steve Chen and Chad Hurley. (There was Karim, but normal people have short memory to remember the third person.)

Then, we ask why 2 founders? Why not alone or why not many?
Y Combinator clears states that a startup seems to be overwhelming for single person entrepreneur on their webpage. To make an idea become a real business, there needs multiple skill sets, mostly a combination from multiple talented entrepreneurs. So that’s why single-person startup is hard to find. (There are exceptions, always.)
Then why not many founders?
A startup is lack of financial support during the most of their bootstrap period. Founders work with their passion, not for monthly paychecks. This makes extremely difficult to recruit co-founders. It is also why a startup started by roommates, classmates, friends, etc.
Also, most startups are innovative companies, that only a few people share same vision and understanding. Vision sharing is an important factor to keep the startup survive during the bootstrap period. it is also very difficult to understand and share the founders’ vision.
So, Number 2 (or 3, and 4) seems to be a realistic and optimal number range of founders for innovative tech startups.
If you think of launching your startup company, you should ask “Where is my partner?”.