There are 330 million people in the United States. So if Mike Lindell really has proof that 300 million Americans were involved in election fraud to install Biden as President, he's talking about 91% of the population, including a whole lot of children. The adult population of the U.S. (18 and up) is only 209 million, so at least a third of his election fraudsters are kids.
Plus, if 91% of the U.S. population were working to make Biden president, why would they even need to engage in fraud? Lindell is inadvertently claiming that the vast majority of Americans would rather have Biden president than Trump. That doesn't exactly help his case.