Is it true that their search algorithm was developed by over 15000 people? I thought it was developed by only two guys.

This isn't a Linux Question! However...

Where Google originated from and where it is now are two different things.

The origin of the Google Search Engine's algorithm was mainly developed by two guys, however since then google has gone from a garage based cluster maintained by two men to a multi-national corporation worth billions of dollars (US), for such an important company the idea that they would still be using the very same algorithm after all these years doesn't seem likely, so while the original search engine was mainly designed and created by two guys, the algorithm in use today is extremely unlikely to be that same one.