Considering the 12" alone serves 55, I dare say you did have a lot of left overs!
cake size.....# of 1"x2"x4" servings
4...............6
5...............10
6...............12
7...............16
8...............24
9...............30
10.............38
11.............45
12.............55
13.............65
14.............75
15.............85
16.............100
18.............125
20..............150
cake calculator!
http://shinymetalobjects.net/cake/calculator/cake_calculator.cgi
Metria, your calculator is a wonder!! Thanks so much. Just a note, prterrell - the serving size you mention is for wedding cakes. These portions are smaller than party cake servings (although I confess my ignorance about the exact dimensions of a slice of party cake). Metria's calculator lets you decide which of the two portion sizes you want to use. Of course, even so, there were still more servings than necessary in a 12" and an 8" round.
Marianna46 - No, the serving size I mentioned is not just for wedding cakes. A serving of cake is a serving of cake, no matter at what occasion the cake is being served. There is NO such thing as "wedding servings" and "party servings". The 1"x2"x4" serving size is for ALL 4" tall cakes. The 2"x2" slice is ONLY for 2" high cakes (sheet cakes). Both result in the standard amount of 8 cubic inches of cake per serving. Sheet cakes for a very long time were "party" cakes and stacked cakes were "wedding" cakes. This has led to some confusion in thinking that "party" cake servings are larger than "wedding" servings. But, this is a mistake, as my above explanation shows.
Prterrell, I stand corrected. Thanks for the valuable info!
There is NO such thing as "wedding servings" and "party servings".
oh my gosh, why hasn't someone flat out said this before! YES! YES! YES!
A classic explanation that I am saving in my "Save this because you'll refer back to it a number of times!" file!
Regardless of whether you are of the opinion that wedding and party slices are actually of different sizes, the fact remains that most people at a home party are going to cut bigger pieces for their guests than would be served as dessert at a wedding. (How many times have we heard of people who order a cake and then complain that they ran out?) This is why the first step in determining what size of cake should be ordered is to educate the client about what a standard serving size is, and ask whether they plan to stick to it. Explain that while pricing is according to standard size slices, you want to make sure that they'll have enough cake if they're going to be more generous with their servings.
As a courtesy (and to prevent subsequent confusion), I always present this wonderful cake-cutting guide as a visual at the time the cake is ordered, because it represents servings for both 1"x2" AND 2"x2" slices, so people know exactly what they're getting. Many thanks to lexi55033, you can find the guide here: http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1300478 Hope this is helpful to you! 
A standard serving is either 2x2x2 or 1x2x4.
Someone wants to cut a 64 serving cake in 2 pieces? Fine.
They are still charged for 64 servings.
And they have to be shown that if they are going to cut pieces that big, then they have to buy more cake .... 32 cakes, as a matter of fact, if they actually have to serve 64 people!! 
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