It's the number of troops left.
Which often isn't that meaningful because the number you start with varies so much - different troops are different sizes so will have a different number of troops in a squad within an encounter. Within a province all encounters are the same difficulty, but no 2 provinces are the same, and in the tourney and spire the squad size increases a bit for every encounter. So unless you can remember what the number was you started with, or you have an unharmed squad to compare with at the end (if the diamond button is greyed out the squad had zero losses) it's hard to relate the surviving number to anything
More usefully under the number is a coloured bar showing you the proportion of your starting troops which survived the fight.