Statistically 0

Functionally, a literal handful per season which are typically caught and prosecuted.

The right leaning Brookings and Heritage foundations dug into the numbers and found this

"Across the entire country, and utilizing a maximally inclusive estimate, between six and 46 cases of mail voting fraud were identified in each general election"

Table:

Year Total mail voting fraud percentage (out of total mail ballots cast) 2016 0.000089% 2018 0.000024% 2020 0.000051% 2022 0.000010% Average 0.000043%

https://www.brookings.edu/articles/mail-voting-in-the-us-dat...