First of all I have to say that I am not a big fan of SILVERware. It is a pain to clean and polish, it's really expensive, and you use it maybe twice a year at the most. When people look at a registry they typically have a price range in mind and given the choice between buying a full serving set and one teaspoon they will go for the full set every time. Keep in mind that this is my personal opinion, but I prefer to pick a stainless flatware pattern that will look good with both your everyday and fine china.
The number of pieces you need for your table setting is easy because it is the same number of place settings of dishes you requested (I recommend 12). The serving pieces can be a bit more confusing. There are just so many different pieces, and the names of them can be deceptive. Take, for instance, the seafood fork. I don't ever serve the type of seafood that usually requires a special fork, however I use my seafood fork on average once a week. It is great for getting things like olives and cherries out of jars. Long handled ice tea spoons are another useful item that many people forget to register for. I have 4 myself, but could probably use 4 more. They are great for things like jelly and salsa and mixing up chocolate milk. We find a new use for this item all the time. My point is that just because you don't need it for it's intend purpose doesn't mean you won't need it.
Serving spoons are very nice to have. If your pattern is open stock (meaning you buy one serving piece at a time) then be sure you register for at least 2 regular serving spoons as well as 2 of the slotted ones. If, however, your serving pieces come in sets, register for at least 2 sets even if it also comes with a piece you really only need one of such as a ladle.
Before you pick you pattern, take note of how long a pattern has been around and how often the company discontinues styles. After 12 years of marriage my stainless pattern is still available. I know this because I occasionally need a new piece. It's nice to know I can always find it. Some companies have even set aside some of their patterns as being ones they will never stop making. It is nice to know that as long as Oneida is in business I will be able to pick up new pieces.