Number of LED's will depend on the wattage/lumens, and what you're trying to accomplish. A FOWLR will need far less light than a full blown reef setup. For the former the color could be determined largely by taste as whiter light is mainly for human enjoyment, the fish don't really care. In a reef tank I'll leave determining color up to someone else as that's not my main focus. Heat sink configuration is pretty much what you like. If you want to do pendents, you could use a number of shorter heat sinks running front to back, or any other configuration you like. In my hood build I used one long heat sink as I just figured it would be simpler, but any configuration would work, it's just a matter of taste.
Optics are rated in degrees of focus. So a 10 degree optic throws out light in a 10 degree radius. You have to keep in mind though that a ten degree radius is much narrower at a distance from the source of 12" than it is a 36".