12 I persevered in demonstrating among you the marks of a true apostle, including signs, wonders and miracles.

Read 2 Corinthians 12:12 Using Other Translations

Truly the signs of an apostle were wrought among you in all patience, in signs, and wonders, and mighty deeds.
The signs of a true apostle were performed among you with utmost patience, with signs and wonders and mighty works.
When I was with you, I certainly gave you proof that I am an apostle. For I patiently did many signs and wonders and miracles among you.