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.
I know a guy who did this for a job for a company that produced food
https://en.wikipedia.org/wiki/Estimated_sign
his entire job was to allow the company to push up as close to that line as possible without going over when checked randomly
saving them tens of millions a year