Number of start-up founders - Statistics
Since, November 7th, 2007 (approximately)

https://news.ycombinator.com/item?id=77525

I've done some research on how many founders successful technology companies have. Here are the numbers for 100 publicly traded companies

 +--------------------+-----------+
 | number of founders | frequency |
 +--------------------+-----------+
 |                  1 |        40 | 
 |                  2 |        31 | 
 |                  3 |        17 | 
 |                  4 |         6 | 
 |                  5 |         2 | 
 |                  6 |         1 | 
 |                  7 |         1 | 
 |                  8 |         1 | 
 |                 13 |         1 | 
 +--------------------+-----------+
 Weighted average 2.21

Some caveats
The number of sole founder companies is probably slightly exaggerated because sometimes one person is so much more prominent that the others are never mentioned. I did a little double checking though. Another important thing is that some companies were omitted because I knew only that they had several founders but I wasn't able to find out exactly how many. I suspect there are more companies with 4, 5, 6 or more founders.

Unfortunately without numbers about failed companies we still don't know the success ratio of each frequency group, which is what's most interesting for startup founders here. It is well possible that sole founders fail 10 times as often as 2 founder companies, or vice versa. For anyone interested in double checking (yes I'm sure there are errors ;-) here's the full list:

 +-----------------------------+--------------------+
 | company                     | number of founders |
 +-----------------------------+--------------------+
 | ACS                         |                  1 | 
 | Activision                  |                  5 | 
 | Adobe                       |                  2 | 
 | Akamai                      |                  4 | 
 | Altera                      |                  3 | 
 | Amazon                      |                  1 | 
 | AMD                         |                  8 | 
 | Amphenol                    |                  1 | 
 | Analog Devices              |                  2 | 
 | Ansys                       |                  1 | 
 | Apple                       |                  3 | 
 | Applied Biosystems          |                  2 | 
 | Aspen Technology            |                  1 | 
 | Autodesk                    |                 13 | 
 | Avnet                       |                  1 | 
 | Baidu                       |                  2 | 
 | BEA                         |                  3 | 
 | Beckman Coulter             |                  1 | 
 | BMC                         |                  3 | 
 | Broadcom                    |                  2 | 
 | Brocade                     |                  4 | 
 | CA                          |                  1 | 
 | Canon                       |                  2 | 
 | Cerner                      |                  3 | 
 | Check Point Software        |                  3 | 
 | CheckFree                   |                  1 | 
 | Cisco                       |                  2 | 
 | Citrix                      |                  1 | 
 | Cognos                      |                  2 | 
 | Compuware                   |                  3 | 
 | CSC                         |                  3 | 
 | Cypress Semiconductor       |                  1 | 
 | Cytyc                       |                  1 | 
 | Dell                        |                  1 | 
 | Dolby Laboratories          |                  1 | 
 | Eaton Corp.                 |                  3 | 
 | Ebay                        |                  1 | 
 | Electronic Arts             |                  1 | 
 | EMC                         |                  2 | 
 | Fiserv                      |                  2 | 
 | Flextronics                 |                  1 | 
 | Garmin                      |                  4 | 
 | Google                      |                  2 | 
 | Harris Corp.                |                  2 | 
 | HP                          |                  2 | 
 | IBM                         |                  1 | 
 | IMS Health                  |                  2 | 
 | Informatica                 |                  2 | 
 | Infosys                     |                  7 | 
 | Intel                       |                  2 | 
 | Intuit                      |                  2 | 
 | Jabil Circuit               |                  2 | 
 | Juniper Networks            |                  3 | 
 | Konami                      |                  1 | 
 | Kyocera                     |                  1 | 
 | Lam Research                |                  1 | 
 | Logitech                    |                  3 | 
 | LSI Corporation             |                  3 | 
 | Man Tech                    |                  1 | 
 | McAfee                      |                  1 | 
 | Micron Technology           |                  2 | 
 | Microsoft                   |                  2 | 
 | Mindray                     |                  1 | 
 | Motorola                    |                  2 | 
 | NetApp                      |                  3 | 
 | Nokia                       |                  1 | 
 | Novell                      |                  4 | 
 | Nvidia                      |                  3 | 
 | Open Text                   |                  1 | 
 | Oracle                      |                  3 | 
 | Parametric Technology Corp. |                  1 | 
 | Philips                     |                  1 | 
 | Qualcomm                    |                  2 | 
 | Quest Software              |                  2 | 
 | RIM                         |                  2 | 
 | Roper Industries            |                  1 | 
 | SAIC                        |                  1 | 
 | Salesforce                  |                  1 | 
 | SanDisk                     |                  2 | 
 | SAP                         |                  5 | 
 | Satyam                      |                  1 | 
 | Seagate                     |                  2 | 
 | Siemens                     |                  1 | 
 | SRA International           |                  1 | 
 | Sun Microsystems            |                  4 | 
 | SunPower                    |                  1 | 
 | Sybase                      |                  2 | 
 | Symantec                    |                  1 | 
 | Synaptics                   |                  2 | 
 | Tellabs                     |                  6 | 
 | Thomson                     |                  1 | 
 | TI                          |                  4 | 
 | Tibco                       |                  1 | 
 | Trimble Navigation          |                  3 | 
 | VMware                      |                  2 | 
 | Waters Corp.                |                  1 | 
 | Wipro                       |                  1 | 
 | Xerox                       |                  2 | 
 | Xilinx                      |                  3 | 
 | Yahoo                       |                  2 | 
 +-----------------------------+--------------------+