It's all based on time. You have 15 minutes to get 10 troops in from the time the first enters. If some of your troops are killed by enemy fire others dropped after still count as long as they are inside that window.
You can not drop friendly troops to reset the timer or count. You can however resupply the town and reset the white flag.
Bailing will not kill your troops. Once running only enemy fire can stop them.
