I am never successful at redeeming passcodes, maybe I am just always late. I would like to know how many people can redeem a single passcode?
The number of people that can redeem a single passcode is not divulged. Only Niantic knows that information. There are one-time only codes, usually given out at events, or better than average passcodes, like those given out with the introduction of the new portal mods in version 1.28.
The number is estimated to be in the order of a few thousand redemptions, this means that if you don't get the code within a few minutes of it becoming public, you're out of luck.
There are various ways to get these passcodes:
follow notable people from Niantic on Google+ (Brandon Badger, AnneBeuttenmüller et al.). They don't give out codes at regular intervals, but still you can be lucky.
try to solve the puzzles on http://www.nianticproject.com/. The page changes everyday and hidden inside it you can find new passcodes every time it's updated. This is the best way to get a code that works, but it requires to have some familiarity with web-based puzzles and at least a base knowledge of cryptography. DeCode Ingress is a G+ community of dedicated solvers: here you won't get passcodes, but you can ask hint to solve the puzzles and get a general idea of how to solve one, since usually no puzzle is like the others.
follow one of the many communities on Google+ dedicated to passcodes. Ingress passcodes is probably the best known. Pay attention to the rules and be aware that once a passcode gets here you only have 2 minutes at most before it's fully redeemed.
coordinate with your local faction. Usually, especially in the bigger cities, there's a channel set up by your faction dedicated to distributing the latest passcodes. It can be a Hangout group, WhatsApp, paper airplanes, you can pick your preference. Usually passcodes get here only after they've already been posted in the G+ communities, so this means you'll have just seconds to redeem them. Sometimes though you can be lucky and have a fellow agent that is also a skilled puzzle solver that is willing to share the passcodes he finds with you, before giving them to the public.
Avoid like plague the passcode apps, if you're lucky you'll only get already expired passcodes, but some can try to steal your personal data.