1) you can use generally 3 bars on the same screen, with a resolution of 1024x768 (and a 15''). You can use 4 or more bars with a higher resolution. If you have a resolution 800x600, you should think about increasing it: you are letting some money waste.
2) Some bars can overlap, so you can use more than 3 or 4 on the same screen. But I do not like this behaviour; these people pay us money to see their ads... and I consider a steal trying to cheat them.
3) How much can you earn? Well, if surfed 30 hours per month, with 3 bars always active, you should earn about 40$. If you surf more, you will earn more. If you find many referrals, your earnings may have no limit.
4) Someone asked me about money for e-mail. Sorry, I do not like this kind of programs. Why? Simple: for receving a check, you would need MANY referrals. A little example: they send to you about 5 e-mails per week (this is the current average, up to now), paid 5c each one. You need to earn al least 30$ to receive your check: so, you need to receive 600 e-mails. With 5 e-mail per week, you need 120 weeks for receive your money: since there are 52 weeks in an year, you will need more than 2 years and 4 months. If you consider that an average of 5 e-mails per week is quite high, that you could receive even less e-mail, you can understand why there are not e-mail companies in my rank.