What does this number represent?
It's the base size of the model. In Kaelyssa's case, and all other small base models, it's 30mm.

Here's a reference from PPS_Jackson on Reddit where he answers this very question:
https://www.reddit.com/r/Warmachine/comments/6afm9k/comment/dhfgmxg


  That indicates the models base size which is a new addition given how dragoons are now shown as two separate cards.