How many PTC sites / Work At Home sites could there possibly be?

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I am on several PTC or work at home sites. I know that is true of other mylotters. There are so many with new ones every day. How many valid and legit ones can there be out there? I hear about some from other mylotters but I also hear about a lot through clixsense which is one of the PTC sites that I am on. Clixsense advertises some of the old ones like neobux but every day there are ads about the newer ones and brand new ones. I keep thinking "how many can there be"? How many of these are legit and how many are scams? How many are going to withstand the test of time? These sites need advertisers and subscribers. How many advertisers are out there? How many sites can they afford to advertise on? And what about subscribers? There are only so many sites that I can give my time and energy to. And probably most potential subscribers are like myself. We have to make choices. Just wondering about this when I click on the ads that I get from clixsense.