Keep an eye out for them at all times. Every use case is a potential requirement, and until you have captured a requirement, you cannot plan to deal with it. Some people list and discuss the use cases first, then do some modeling. I sometimes do that, but I've also found that conceptual modeling with users helps uncover use cases. I recommend that you try it both ways and see which works best for you.
Different designers make use of use cases with varying degrees of granularity.
In a recent project of about the same magnitude, I had more than 100 use cases. I prefer smaller-grained use cases because they make it easier to work with the commitment schedule. However, too many use cases can be overwhelming. I don't think there is one right answer at the moment, so be flexible and work with whatever seems comfortable. Where to Find Out More I think the world is still waiting for a really good book on use cases.