Pest Rescuer's first ability checks at the moment it would trigger to see if you control any Pest creature tokens. If you do, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you control any Pest creature tokens at that time, the ability won't resolve and none of its effects will happen.

If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and the last ability of Pest Rescuer applies only once.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Pest Rescuer's last ability will apply twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will apply only once.

If you control two Pest Rescuers and you would gain life, you gain that much life plus 2. A third Pest Rescuer has you gain that much life plus 3, and so on.