What is the appropriate ratio of medics?

What is the appropriate ratio of Medics to team size for most non-pro games? I'm fairly sure one medic in twelve players isn't effective.

A straight ratio isn't the best, because there are some classes that the medic doesn't synergize well with (engineer, spy). A better metric would be 1 medic for every 4-5 offensive classes, so 2-3 medics on a 12 person team.

There's not a set ratio, but they key is to at least have 2 because medics are exponentially better when they can heal each other. 1 medic is bad not because they can't heal as many targets (though of course that's bad), but because when they get hurt, they have to back off the line to heal themselves.

I'll add one more answer: it depends on what stage of the game you're in and what mode you're playing. In Attack/Defense, if you're on BLU, it's very helpful to have more medics than usual at the beginning of the round (as long as each of them has an offensive player to buddy up with) because they can all build up an Ubercharge before the round starts. Then when the gates open they can Ubercharge more than one offensive class player at the beginning, or extend an Uber started by another Medic on a Heavy or Pyro (which is what I usually do, RED is usually ready for Ubers out of the gate with their own Uber but they don't expect yours to last twice as long). I'll often start a round as a medic and then switch to another class after that first uber is spent, assuming there's another medic on my team.

In CTF, the Quick Fix is handy for buddying up with Scouts, since it speeds up the Medic to the Scout's speed and its super-healing Ubercharge doesn't prevent the Scout from grabbing the intelligence (which the normal Medi-gun uber does).