Does Mogg Sentry gain +2/+2 for each spell?
Mogg Sentry becomes a 5/5 until end of turn. Mogg Sentry's ability triggers once for each occurrence of "opponent casts a spell."


  603.2c An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 509.4.