Rule 702.15e

If multiple sources with lifelink deal damage at the same time, they cause separate life gain events (see rules 119.9–10). Example: A player controls Ajani’s Pridemate, which reads “Whenever you gain life, put a +1/+1 counter on this creature,” and two creatures with lifelink. The creatures with lifelink deal combat damage simultaneously. Ajani’s Pridemate’s ability triggers twice.

Find this number in the current Rules