AS MUCH AS YOU WANT - WHENEVER YOU NEED IT
If the goods and services you use and the goods and services you provide do not balance out, you may run out of WTF notes.
If that happens simply switch on your PC or go to the copier shop, and print as many WTFs as you need.
If you spend your WTFs unwisely, or they are stolen, your only loss will be the price of replacing your WTF notes, perhaps 5-15 WTF-cents, euro-cents, dollar-cents or UK pence per page full.
Tip: Don't try to fit too many notes on a page, or when you cut them up your WTFs will be small and fiddly.