A few states have been all mail-in for years. 10s of millions of votes were cast by mail in 2016 and 2018. There were certainly more this year, but only like 4-5x.

(It helps that the most populous state is mostly mail-in).

HN is throttling me, so here's some more info:

https://www.ncsl.org/research/elections-and-campaigns/absent... and https://electproject.github.io/Early-Vote-2020G/index.html have most of the info you want.

CO, DC, HI, OR, UT, and WA are vote by mail for all elections. CA was sort of doing VBM in 2016, but it's more committed to it now. This year, NJ, VT, and NV were also vote by mail as one time things. So in prior years there were more than 30 million mail ballots, and AZ, CA, CO, FL, MI, OH, OR, and WA had more than a million mail votes cast (though some of those were likely classified as absentee in MI and OH).