5000 messages doesn't seem like a lot. Is that per agent or total?

It is total, and you are probably right.

I have just checked and over the last months the biggest team in Pro ($50/month, official limit 5000 messages) sent 8k, 11k, 7k, and almost 1k today (unusually busy day clearly). Right now they have a few dozen agents live. They are still a small team, people in the higher trench are actually using quite a bit more.

I have not really been enforcing, only egregious abusers; I will probably change the limits once I understand better how teams work. I want this to actually be reasonable and sustainable.

[edit] Updated.