Usually, pts have an order that says, "02 @2L, titrate/keep sats >92%".
But, I'm not going to wait for an order if my pts sats drop. I'll call the Dr when I can, or have someone else call if I don't have an order, but I'll just place the O2 right away.
You wanted to know how much to put on, or how we know how much to put on? The answer is, however much the pt needs. If his sats drop to 85%, and you put him on 2L, and they are still in the 80's, then you trurn the O2 up.
(Of course, your actions are based on the pts history, reason for admission,etc, etc...)