Usually 6-8 jets because if you have more than 8 jets it will require a second plumbing loop with a separate pump. Even at 8 jets, you will need a fairly large pump and fairly low head loss plumbing setup.
Were you planning to have only one pump for both the jets and the pool circulation? If you have a separate spa pump with large plumbing combined with low flow jets, you can sometimes get more jets.
Have you picked out your jets yet? The flow rate requirements vary by the type of jet so it is important to pick those out first before deciding how many jets. Some require 25 GPM each which means that you would only be able to support about 4 jets. Others require 15 GPM.
There is a section in the hydraulics 101 sticky in my signature that covers some of the design criteria for spas.