If you didn’t gain life during the turn before the end step begins, Regal Bloodlord’s last ability won’t trigger at all. Gaining life during the end step won’t cause the ability to trigger.

You create only one Bat token, no matter how much life you gained.

Regal Bloodlord’s last ability checks if you gained life at any point during the turn. It doesn’t matter if you also lost life or whether your life total is greater than it was at the beginning of the turn. It also doesn’t matter whether Regal Bloodlord was on the battlefield when the life gain happened.

In a Two-Headed Giant game, life gained by your teammate won’t satisfy Regal Bloodlord’s last ability, even though it caused your team’s life total to increase.