The ability triggers just once for each life-gaining event, whether it’s 1 life from Soulmender or 8 life from Meditation Puzzle.

Use Wall of Limbs’s power as it last existed on the battlefield, including any +1/+1 counters it had, to determine the value of X.

If you gain life at the same time Wall of Limbs is dealt lethal damage (probably because you control a blocking creature with lifelink), Wall of Limbs’s triggered ability will trigger, but Wall of Limbs will be destroyed before the ability resolves.