As many as your energy system can comfortably handle.
I know that can seem like a “non-answer,” but it really is the accurate answer. Everyone’s energy systems are different. I might be able to play as many fields as I can cram into 24 hours (and I often do hehe) whereas another forum can only listen to a part of 1 field before they start feeling the effects of overdoing it. (For a list of some of those effects, check out our Official Sapienmed FAQ (Updated).)
It also depends on which fields you’re listening to. Some fields are more “taxing” than others. Some of that has to do with our own issues, so that level of “taxing-ness” can be improved by a rejiggering of our stacks.
Additionally, creating permanent physical changes on our bodies is often usually the most taxing of goals. So, the guideline there is, until you know how your fields affect you, it’s recommended that you not play more than 4 fields directed towards creating a permanent physical change in yourself.