Do we actually have any numbers on this? I know it’s not really an answer but if people stick to “jack of a trades” tanks they wouldn’t be having such a hard time with all the bs in this game.
You can ball park it by queue time ratio between different roles… But it isn’t an exact metric…
Most of them go to open Q instead of doing role q cuz u can have more then one tank then lol
Depends on latency, your mmr and region.
Some places like U.S damage role are really popular. Others supports can have some space but due the number of heroes often damage have the biggest queue. But roughly most places tanks are the smallest crowd.
On open, I find most my matches with 1-2 tanks. Sometimes 3. Rarely 0 or more than 3. Although most of my matches on open are dynamic to the point of most players swapping about 3-4 times during the match.
Also I feel my open matches more balanced. These days I don’t play much, but for sure I only touch tank on open or to actually help a friend in about 1-2 matches at best.
Too low. But bringing back 6v6 is still a good idea 
(not)
Who are you talking about when you say “jack of all trades” tanks?
Look at the bottom of the t500 leaderboard for tanks and compare it to DPS and support.
There’s your answer.
If you cannot physically check, bottom of tank t500 is literally diamond 2… bottom of DPS and support is m4 and m5