I have a dummy cake and 12x18 sheet cakes for the guests. There are 155 guests. I made 3 layered sheets- if I use the wedding cake template I'll have too much- if I use birthday cake template I'll have 162- would the 162 be cutting too close; so to speak?

the wedding slices seem so small- has anyone done that before? Used the birthday party cake cutting for a wedding? Help its tomorrow

You have 3 sheets. 155 guests plus a few extras makes 165 (3 x 55). With layers they must be pretty close to 4" high, right?

Cut the first two sheets into 60 pieces each (10 x 6). That gives you servings pretty close to the birthday cake size. Cut the third sheet according to the crowd...if the pieces have to be fractionally smaller, nobody will be sitting there with a ruler.

I have baked "serving" counts somewhere in between the two Wilton lists for wedding cakes, but I have always asked about what other sweet food was being served.

..... I made 3 layered sheets......

Are the sheets 2" thll/thick OR 4"?
That will help decide how to cut it.
With 155 guests - some will not show, others will not eat cake and still others will try for 2nds.
Each 12x18x2 sheet will serve 54. Each 12x18x4 will serve 108 in 1x2x4 servings. Either way the guest is getting the *SAME* amount of cake - it's just how it's presented.

That's nearly 300 1x2x4 inch wedding slices:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

8 cu. in. servings are sufficient. You could get away with one double-layer 12x18 kitchen cake and a double-layer 9x13 kitchen cake.