As far as I know, you can carry as much insulin as your check luggage can hold.
As ar as carry ons go, there is a seperate line where you can get medical necessities checked. On any number of occasions, I have not had my insulin checked. I've just run it through the x-ray in my carry-on luggage and that was that. On the few occasions where I've had more than 6 oz for extended trips, I've never had a problem putting it all in a carry-on.