*It is impossible to calculate an exact figure, since states use such different procedures. However, about 1% of mail-in ballots are rejected nationwide. Most are rejected because they are late, but the second most common reason is for missing or mis-matched signatures. Using a low estimate of 0.1% of ballots being rejected for a mismatched signature, we can estimate that 1,300 of PA's 1.3 million ballots would have been rejected for signature mismatch in 2020. That's 1,300 eligible voters who voted in good faith.
Read more here: "Signed, Sealed, Delivered - Then Discarded" , The Atlantic October 21, 2020