PTC sites do seem to be more tricky than other forms of sites for making money mostly for the reasons that sanglee gave. If you are trying to make money on these just by your own clicks, then it does take quite a while to accumulate enough to cashout, and even then, the cashout amounts are only a few dollars. So for these types of sites, the only way to really be successful is through your referral clicks, not necessarily your own clicks. Many of the PTC sites offer "rented" referrals which allow you to get some referrals under your account without having to actively recruit them, but for a price usually for a month to begin with and the option to increase their rental period later (and usually the longer lease times that you purchase will provide a little cost savings than just continuing with 1-month rentals). These rented referrals are good in that they do provide you with an income above what you'd be able to get on your clicks alone, but because of their rental costs as well as their own activity on the site may eat away at any profits that you're getting from their clicks. Thus if you do get rented referrals, then you'll need to spend more time just managing those referrals (you can usually "recycle" referrals that you think have a low clicking rate (making their rental costs more than their earning potential) to get new (and hopefully better) referrals for the remainder of the rental period, but at a cost for recycling them. So it can quickly get complicated in determining when to recycle, for recycling too much will eat away your profits just in recycling costs, but not recycling enough will also decrease your profits in rental costs outpacing profits from those referral clicks.
I know that some people don't really use PTC sites as a place for earning money, but rather treat them more like a traffic exchange where all of the money that they earn on the site is used to fund ads on that site for other sites (like FC) where they earn more money from referrals to those outside sites. And instead of just providing a referral link to a different site, they sometimes will link to their own person blog / website that showcases multiple sites for making money online, and provide the referral links off of their personal web space.
Right now, I'm active on 7, although there are a few other sites that I have accounts with but I am just not staying active with. But there really are 4 that are most important for me in that I have rented referrals on those, and in order to get credit for those referral clicks, I need to also click at least a few ads as well. So if I'm really pinched for time, I'll just hit those 4 sites and do the minimum number of clicks. Usually I just use the money that I earn on the site to start building up a referral base (by renting) and only after that do I actually cashout money for myself. Usually if I find that I'm progressing too slowly on a particular site, I may just stop investing with whatever referrals I have, and just cashout what I can before leaving the site. There is only one PTC site that I've called a true success in that I've built up 250 rented referrals that have a good average (I think in total I get an average of over 1 click per referral, which with getting 1-cent per click means I earn about $2.50 per day), and even after spending some earnings on the referrals themselves (eg extending their lease and / or recycling those referrals that average less than 1 click per day that I've had for at least 10 days (ie my recycling strategy for that site)), I still am able to cashout every week or 2 for about $10. Again that is much less than a true job, but I also don't have to spend a whole lot of time on the site either (eg if I'm pressed for time, I can spend like 15 minutes on the site (that includes both viewing the few ads that I need to to get full credit from my referrals as well as checking over my referral list), so in that sense it actually is a fairly good earning average.
I wouldn't really put a number of sites to be on as being an ideal number, but rather the quality of the sites that you're on should be the limiting factor. For example as I've mentioned some sites have terrible click averages for their rented referrals, so for those, I'll just cashout what I can and then leave. But for others if I see a good benefit for being with a particular site, I'll stick with it and make it into one of my regulars.