Flash When Selfless Squire enters the battlefield, prevent all damage that would be dealt to you this turn. Whenever damage that would be dealt to you is prevented, put that many +1/+1 counters on Selfless Squire.
Selfless Squire's first triggered ability prevents all damage that would be dealt to you this turn after the triggered ability resolves, even if Selfless Squire leaves the battlefield.
Selfless Squire's second triggered ability doesn't trigger only when damage is prevented by its first triggered ability's effect. Any effect that uses the word "prevent" will cause it to trigger.