Tales of Mystic Tournaments says that 1L will pay for a common feast for 350 knights, a quality feast for 175, a superb feast, a grand feast, a regal feast, and an imperial feast.

There are a number of examples of feasts, describing "a regal feast of 7 courses" in one place and "a grand feast of 7 courses" in another, but there is also an example of a feast of 12 courses.

So my question is, how do you figure out how many courses are in a feast?

This is way below the resolution of my campaign, so I would simply ignore the exact number of courses anyway. But as a rule of the thumb IMHO, if the courses are rare and expensive, there would be less of them, but the whole would still be considered regal / imperial whatever since the expense and rarity would make up for less variety.

I do as Morien does and ignore this. But a more expensive feast could either have more expensive food or more course imho. The number of courses is not really set with the price you pay.