Live? How live?

Store bought yogurt claims to contain live cultures, but I wonder how it compares to that of home made yogurt. The thing is, when you make yogurt at home, you have to consume it within a few days because the action of the bacteria turns it more and more sour, until at some point it's inedible. But that doesn't seem to happen with store bought yogurt. Or if store bought yogurt does become more sour, it happens much more slowly. So does home made yogurt have a larger bacteria count? I know home made yogurt has a much more diverse set of cultures as opposed to the single digit number of bacterial cultures that store bought yogurt has which are mandated by the government. Or in the production process, are the bacteria in store bought yogurt attenuated somehow? Now, the good bacteria is supposed to be one of yogurt's main plus points. So, is homemade yogurt much healthier than store-bought yogurt?