I think offshore is the largest segment - there are close to 1000 helicopters operating in the Gulf of Mexico. EMS is close to that size, perhaps, but it's spread out.
The way most of us built time is with the military. We're starting to see more civilian-only pilots, but they're still the vast minority. It costs so much to get a helicopter commercial license, and then get the required time, that it's just not economically viable, given the low pay involved. Helicopter pilots, on average, make less than fixed-wing pilots.
Absolutely the only reason I'm flying helicopters is the free training and flight time I got from my rich uncle, Sam. All I had to give in return was several years of being a target for people who didn't like U.S. military helicopters.