There are many variables! As Dr. Spa has mentioned, jets are rated at a given flow. If you provide them with the appropriate flow rate, they will draw in air, mix it and give you a good massage. If you under or over feed them they will not - plus you may get pressure backing up and causing problems with plumbing by vibrating, making noise, or even loading the pump incorrectly.
A rule of thumb that I have used is to allow one fourth of a HP for each jet, but I used to install jets with removable orifices so I could 'tune' things after the fact. I don't know if they even offer jets like that anymore! Now they have removable nozzles of a variety of designs from straight to eyeball to spinners, you name it! You can find flow charts for most pumps at the manufacturer's web site, and just about all jets should be listed right next to specs which include the size of whole you need to drill, the size of plumbing you need to use, and the suggested flow.
Some Jets now have built-in valves, which is cool because if you don't want to do your homework you can simply pop in six or eight of those, and adjust the flow as you enjoy the tub by simply shutting off or turning on jets until it feels the way you like it - effectively going from a low-pressure (all jets running) to a real pounding (only one or two jets running). If you find that the best mix is to have four jets on, then you still get to chose which four jets you enjoy, and of course that can change from soak to soak, and from minute to minute as you move about from seat to seat.
