Druids - It's been my experience that no more than one resto druid plays WoW per server. And they're Alliance. If you have two, hug them tightly and bring the second one on fights were there's no aggro wipe (i.e. not Hydross or Morogrim).
Priests - With their jack-of-many-trades spell selection, I try to always bring at least two. If you have a lot of priests, more than other healing classes, it's ok to bring three. On fights where low aggro healing is critical, more is better. The more you have, the more you should encourage them to coordinate their specs. You only need one with imp spirit per raid. (I just wanted to say "more" one more time.)
Paladins - Ok, I'll be blunt here. I use them for filler. They stack very well buffs-wise and millions of 1k-1.8k heals flying around a raid never hurt anyone. Ideally you'll have 3 in a raid to cover all the buffs (this includes any ret/prot ones you might have). If you have tons of pallies, don't be afraid to bring 4 total. Like priests, the higher the number attending, the better they need to coordinate their specs. There's no excuse not to have kings, imp devotion/conc auras and imp wisdom in a raid.
Shamans - You really, really need at least one, but two for some of the fights is great, especially if there's a ton of AoE damage. From an overall raid standpoint, you need 3 total for the dps buffs, so be willing to work with the raid leads on this. In other words, you may need to run with extra resto's for the good of the group. Hopefully you have some enh/elem around if you're low on resto, though. If you don't have any resto at all, you can limp along with a CoH priest for at least a little while, but you're going to need those totems.
Example Raid Groups
These are in order of most ideal to least, imo. That doesn't mean one at the bottom can't work. You'll just be sacrificing some synergy. I'm also assuming you keep the same group for the entire evening. If you swap people out, then the perference changes based on the fight. And these are "Ideal" and may not be representative of what you actually have to work with.
From Most Ideal
Priest (2), Paladin (3), Druid (1), Shaman (1)
Priest (1), Paladin (3), Druid (1), Shaman (2)
Priest (2), Paladin (2), Druid (1), Shaman (2)*
Priest (3), Paladin (2), Druid (1), Shaman (1)
Priest (2), Paladin (2), Druid (2), Shaman (1)
To Least Ideal
*Better than the ones above it if you have a ret/prot pally, too.
Now it's your turn to explode in anger that I just listed your raid comp as "Least Ideal". Hit up the comments below with your take on how many of each class is best. I'm expecting lots of inconsolable rage. Don't let me down!
Next week I'll use 500 words to describe the depth of complexity that is paladin healing. Stay tuned!
Marcie Knox has been healing lead for over a year, including old school AQ40/BWL/Naxx. She has suffered through holy priest and now basks in the glory that is healadin. Her pally is currently dodging waves of mobs in MH and she totally killed all her constructs in BT. Like killed dead.