Most use a 50 shot with boost, it's enough to cool the charge without getting too crazy. Works well with a centrifugal, gives a good boost in midrange torque and the centri will take off where the nitrous power curve starts to taper off
With a build bottom end you could get away with a little more. Thing to remember with nitrous (especially when you're combining two types of power adders) is that it isn't like most power adders in the fact that it makes the most torque at lower RPMs. Unlike superchargers or turbos, with nitrous you're getting a set horsepower level because of a set level of oxygen content with the shot, in other words the horsepower gained doesn't vary with RPM. Obviously it takes much more torque to make 100hp at 3500rpm than 100hp at 6000, which is also why it's a BAD idea to spray at low RPMs. That's why dual power adders can be risky if not done right, put a big shot on a car that already makes a lot of power at low RPMs (Kenne Bell blown car would be a good example) that could be disastrous if you got greedy just due to the stress of the huge torque output. With a centri car you can get away with it a little easier.