I love Microsoft. No where can you expect a definitive answer.

The one article will tell you that the acceptable limit for site collections per web application is 500'000 (http://technet.microsoft.com/en-us/library/ff758647.aspx) and the next article in the same series will tell you it is 250'000 (http://technet.microsoft.com/en-us/library/cc262787.aspx)

Best of all, they both use the same wording!

"An example of a supported limit is the number of site collections per Web application. The supported limit is 500,000, which is the largest number of site collections per Web application that met performance benchmarks during testing. "

vs

"An example of a supported limit is the number of site collections per Web application. The supported limit is 250,000, which is the largest number of site collections per Web application that met performance benchmarks during testing."

In the end these limits are based on benchmark testing. I.e. when the performance of the farm starts falling below an acceptable level. It is not saying that the farm is running at top speed with 249999 site collections and then drops off. No, it is a curve. So imagine the performance curve starting high at 100%, dropping to 95% at 100'000, dropping to 90% at 250'000 and dropping to 80% at 500'000. The key question is, what is an acceptable level of performance. Looks like the author either changed his mind from one article to another, or two authors differed. In the end both are right, I guess. Because only you can define for yourself what acceptable performance is.

And don't forget, these benchmarks are tested in isolation. so when you store tens of thousands of 50MB documents in a document library hundreds of times per site and thousands of times per site collection, the 500'000 benchmark will never hold up. 50 would sound more realistic.