From zero to ten a week. Average one a week probably. Never really looked into it to be honest. If it is the higher number then it'll be a few from one client. For example, I have 4 to do next week and then I quoted one tonight who had 8 rugs. I'll be doing those next week too but that amount is rare for me.
I only really off site clean them when its spring/summer as its easier to do and dries quicker. Doing just one rug is a pain in the Harris!
If your able to get a constant 5 - 10 a week (the more the better) and charge at least £90 a rug (the more the better) then its worth investing in a unit with a proper set up.