How many scenarios would be ideal to select?

Typically, three types of scenarios could be considered:
(1) A pessimistic scenario (or analog), where both the p and q parameters, and perhaps also the eventual market potential, are likely to be low,
(2) An optimistic scenario where either, or both p and q, are high, and possibly the market potential is also high, and
(3) a middle-of-the-road scenario where one or more of the three parameters, p, q, or market potential are around their average levels (say p around 0.03, q around 0.4, and eventual market potential is less than 40% the size of the target population).

You can also consider specific scenarios, such as “customers will quickly try the product through free trial offer” or “the installed base will switch slowly to the new technology.”