How Many Spaces Do You Need?
This may be the biggest question to answer once you start using Spaces. While having more spaces makes it easier to compartmentalize everything you're doing, there are times when more isn't always better. Navigating through as many as 16 spaces can get tiring, for example, if you're only actively using two or three.
Since spaces are dynamic, you can play around to see what works best and make changes as you need them. There may be times when you only benefit from two or three spaces (such as when just surfing the web, chatting, and checking email) and others when you need six or eight (working on letters in Word, a spreadsheet in Excel, keeping tabs on email, listening to music, etc.). Again, the fact that Spaces is dynamic is really helpful.
One thing to keep in mind is that you will probably always have a few core spaces that you rely on all the time. Keep these in the upper left spaces (i.e. those that are least likely to disappear if you remove rows or columns). This can be particularly important if you assign applications to spaces so that you can always be sure where those applications will launch.
After you've used Spaces for a while, you'll get a feel for how you typically group applications. You may even have an idea about this before you start using it. This sort of experience will help determine not only how many spaces to use, but also which applications to assign to which spaces and how to group them together in a way that works best for you.
For a great task-based reference that you can use to learn every aspect of Spaces, as well as other new features in Mac OS X 10.5 Leopard, check out Maria Langer's new Mac OS X 10.5 Leopard: Visual QuickStart Guide. With plenty of screenshots to clearly illustrate techniques, this reasonably priced guide is a great reference to the essentials of Mac OS X Leopard.