I'm wanting to make a druid, but everyone says "Balance sucks, OOMkin, blah blah blah.."

I've never tried healing, I bet I could do it in Heroics but I don't know if i could heal 25 people in Naxx or OS, or VoA etc..

So, is it veryy hard to heal raids? As a druid, of course.
And how many healers are in a 10 or 25 man raid?
AND, Do the healers only heal the 5 people in their group out of the 25 or 10, or do they heal everybody.. cause that would suck...