Depends on how many ballots you sample, but the number you could change would only matter in an extremely close election and in my state, if a vote is within 0.

Depends on how many ballots you sample, but the number you could change would only matter in an extremely close election and in my state, if a vote is within 0.5%, it triggers an automatic recount. These systems have layers of auditing and validation to prevent these errors whether intentional or not.