The amount of testers is not an issue. It is the people that want to see a perfect product released at exacly the time that it was intended to be released.
NOW, we all know that programs are inherently buggy, even MAC OS has its bugs, as for PC WINdows, that is a completely different story, since a lot of the hardware configurations, as well as any "tweeks" or registry modifications *in windows*, can affect a systems behaviour.
Even MAC systems can be tweeked to a degree, I do believe.
As for adding people to test, more hands in the mix only confuses things.