If I have 2 ten inch cakes stacked and 2 eight inch cakes stacked, is that enough cake for 30? I'm at crunch time and starting to panic.
It sounds like more than enough to me, but check out Wilton's cutting guide:
I am assuming that the total height of both of the cakes is 4" and that they are round cakes, not square.
The 10" serves 38 and the 8" serves 24 (1"x2"x4" servings), giving a total of 62 servings, so you have enough for everyone to have 2 pieces and have cake left over.
I made a 8inch with a 6inch stacked once. I don't sell, just make them for practice and fun. I cut it up using IndyDebi's method and come out with 33 slices.
You'll have plenty. A suggestion: when you or the host starts cutting the cake, start cutting the larger tier first. Then whatever cake is left over will be part of the smaller tier. Easier to store a bit of a little tier than a large one.
I use Earlenes chart (not sure what the website is, it's bookmarked somewhere else on my computer but a search should bring it up with no problem for you) so an 8" has 20 servings and a 10" has 30 servings. I think you will have plenty no matter which chart you use with those sizes.
According to Earlene a 9"/6" serves 30 and a 10"/6" serves 38 so yours will serve well over 40.