Clark there is no average per game for most of us...unless you are Ref18!
For me in the last 5 seasons I've called 3 T's on coaches...2 of those were on the assistant, the second before I finished reporting the first. My partners have called three, all on the head coach.
My rule of thumb is handle the situation early and don't leave yourself in the dilemma of should I call it late in a close game.
You will learn how to talk to coaches in answering their questions, and how to handle them making statements. You will develop that line that they cannot cross and the line to warn if they are approaching this line.
Whenever you do talk with coaches the less you say the better. Always stay calm and professional. Don't make threats. Don't warn TWICE, if you have drawn the line don't erase and re-draw it. If you might have missed it or it was close don't BS them, tell them that it was close or you might have missed it. If you did not see it, it's not so hard to say you'll watch closer for it.
Remember, all coaches just want to be heard, so acknowledging and addressing the issue can head off a lot of trouble. The ones that can't handle your way of addressing the issue will T themselves.