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Mogg Sentry becomes a 5/5 until end of turn. Mogg Sentry's ability triggers once for each occurrence of "opponent casts a spell."
To play a non-land card is to cast it.
You cast two spells, so Mogg Sentry's ability triggers twice.
Each resolves to create a continuous effects granting itself +2/+2. As such, Mogg becomes a 5/5 until the end of the turn.