It depends on the depth of your pans and how large you will cut the pieces:
Here are some guides:
Remember if they are inviting 150 people -- not all will eat cake, I often see figures mentioned here of making enough cake for 70-75% of the people. Some are dieting, some don't want sweets, some leave early, some would rather drink beer.
Anyway, if you make the top tier as a gift for the couple to take home, figure the rest of your cake sizes to feed the attendees.
A 14" could feed 78 pus a 12" could feed 56 that would be 132 slices. If the cake sizes you are making won't feed as many. Make another tier and have it in the kitchen for additional slices.
Another option is to reduce the sizes of the wedding tiers and make a groom's cake to have additional slices.
Let me know if you have any questions. I'm not an expert but have made about a dozen wedding cakes.
Good Luck, Dee in AZ