Does anyone have any experience ordering Sam's baby's breath?

A stem is exactly what it sounds like - a single stem. So baby's breath will have like a main stem (like the trunk of a tree) that splits into branches with the flowers on top. 100 stems will be like 100 tree trunks, each with some branches and a puff of flowers.

Baby's breath is pretty dainty and compact, so 100 stems is not a whole lot.

I ordered baby's breath from costco and was really happy with it. Like Kari said each stem will have a varying number of little branches that split off from it. One way to estimate how much you'll need is to buy some baby's breath from a local florist or grocery store and do a mockup of whatever you'll be creating, and then count up how much you used, then order that much plus a little extra as a safety net.

What will you be making?

I did a little mockup this weekend, I want to do just some poofy baby's breath on a cute vase - I spray painted one black because I like the contrast. I used (based on your description) a little less than 10 stems but I'm thinking I want to use more so I'm going to order about 150-200 stems. I want to make about 10 of these centerpieces.

Those are super super cute, I think they already look really full and lush with just what you have there

So Cute! You can also order baby's breath from fiftyflowers.com I personally have not ordered from them yet, but I hear great things about them. I will be doing very similar centerpieces fro my wedding. I love simple centerpieces!