This is how they work:
1. They start with 8 hitpoins and get more when they split or level
2. When they are at 9 units, no more splits
3. If they get injured their number will drop (like any unit), but they can heal back to the number of units that you have split them to.
Example:
You have a full health group of 9 units with 240hp. After a tough battle you have 4 units at about 100 hitpoints. If you kill someone, now you get no split. If you let them heal for a while you have 9 units again.