Portions
Jen will give you lots of information about how many tiers you'll need when you contact her for a quote.
Here's a quick guide to the number of portions you'll get from a round tier. You'll get more from a square cake!
The portion numbers I'm quoting here are 'finger portions' which are the smaller 1"x1" slices. If you would like to give your guests bigger portions (a 'dessert portion' is 2"x1") just divide the number of portions stated by 2.