You can't just look at the numbers needed to change an election after the fact because whoever committed the fraud wouldn't have perfect knowledge on how much fraud to commit. The votes also can't just be dumped into a single machine, voting center, precinct, or even county because that shift in behavior would be noticed. Therefore this malicious actor would need to be able to add hundreds of thousands of votes state wide across several states in order to change things. Each state will have its own equipment and procedures and those can even vary within states. Once again, this would need to be a very widespread fraud that likely requires dozens of different attack vectors all executed in coordination. It is possible, but very difficult to pull off successfully.