I think it's more accurate to say that a percentage of people believe they will benefit by repeating claims that the election was rigged.
It's difficult to truly believe that an election was rigged, by the other side, when your side was ultimately responsible for its administration.
In a mid December CBS news poll, “82% of Trump voters say they do not consider Mr. Biden[‘s win] legitimate.” The vast majority of trump voters isn’t the same thing as half the country.
https://www.cbsnews.com/news/cbs-news-poll-most-feel-electio...
Seems like most voters were still holding out while there was some hope of a viable legal route to contest the election. But a majority of Trump voters, at the time of the poll anyway, made a clean break from contesting the election after the electoral college vote.
And to be really fair, half of the country believed neither of those things. There are plenty of people who voted for Trump who believe that he lost fair and square, and ditto Hillary voters in the previous election.