Girl, you have come to the right place! There's a great little cake calculator that was posted on here by a CC member named metria. Here's the link:
You can calculate the number of servings a cake will give you either by volume or from the Wilton cake charts on this site. You can also find out how much batter and icing you'll need, so it's a very helpful tool. I use it all the time and cannot thank metria enough for coming up with it.