The simple answer to how may divers there are in Coz a month is...A LOT!!! Gee, maybe that's because it's one of the best places to dive in the world!!
I rarely see groups from other boats while on the reef though, with the staggered pickup times, (my op picks us up at 7:30 some don't pick up unitl as late as 9:00) and different amount of pick up stops the boats make at the resorts, the time that individual groups hit the water varies, this seems to help keep everyone seperated pretty well. not always, but it helps.
The number of divers isn't really the problem, it's the number of divers that don't follow the rules that's the real question and that statistic will never be taken.
For your own personal safety make sure you:
1.) dive with a reputable dive operator, the best in Coz are constantly being recommended here.
2.) FOLLOW THE DM'S INSTRUCTIONS!! this seems to be a major problem with some people. I've seen and heard of so many people that just think they know more than the DM and do whatever thay want once they are in the water. these guys dive these waters every day! I think they know more about what's going on there than you do.
3.) always know the name of the boat you are diving off of, just in case you do get seperated or mixed up with another group. that way if you do happen to get mixed up iwth another group, DON'T PANIC, just continue the dive, watch your dive time and air in case they came down later than you did and when you surface let the DM know what boat you should be on and the Captain will radio your boat and they will come get you.
of course one of the best ways of not getting seperated is stay with your buddy and keep YOUR DM, in eye shot.
4.) I feel it's a good idea to review your dive tables even if you are using a computer, it never hurts to go back to the basics every now and then, and if you are using a compter, KNOW HOW TO READ IT! just owning one won't keep you safe.
5.) Safety first, this was drilled into me as a dive student many years ago and is still my #1 rule. I'm not sure it is stressed as much as it should be by many of today's instructors. I've personally witnessed people doing thier checkout dives in Cozumel and have noticed many things the instructor should have given them for last minute instructions and didn't.
6.) don't worry about how many fatalities a dive destination has a month, just make sure you don't become one yourself. remember everything you learned getting you certification and review it often.
Ok, off the soap box....now go on vacation and have good time!!