There can be no way of saying how many servings you will get from a cake, as the portion sizes will vary due to a number of factors, mainly the occasion and the generosity of the person cutting up the cake!
A wedding cake is traditionally given as a simple taster and it is becoming increasingly popular to 'cut the cake' during the evening reception and serve along with the evening buffet, generally portioned up into 1" x 2" size for sponge cakes and 1" x 1" size for fruit cake (fruit cake tends to be a lot richer, so you need less cake per portion).
However at some weddings it can be served as a replacement for a dessert during the wedding breakfast and therefore you would want to serve a larger portion. Christening and Anniversaries also tend to only offer a taster sample of cake whilst it is more traditional to have a decent sized slice of birthday cake!!
When considering portions other points to remember is that you get more portions of cake from a square cake than a round one and if ordering a carved novelty cake these are only a very general guide as the way the cake has been carved means that not every slice will have an equal portion.
Below is a rough guide to give you an idea of what size cake you might need to meet your requirements:
6in + 9in
8in + 10in
9in + 12in
6in + 8in + 10in
6in + 9in + 12in
8in + 10in + 12in
6in + 8in + 10in + 12in