Too many.. I was over 8000 easy in a month before iMessage, now it's dropped to about 1000-1500 a month.
Actual texts is only a couple hundred a month. Throw in "data-type" texts from apps like iMessage, Viber, Voxer, and What's App, and it's likely in the thousand range.
I'll never pay for an unlimited texting plan with so many great alternatives available.
It seems only a handful send over 15k texts... I wonder if those are teenagers.