It’s too early to know the voter turnout numbers, but you can track the mail-in ballots returned daily with Political Data, Inc.’s 2020 primary election tracker. As of yesterday, of the over 16 million ballots mailed, 27% had been returned. Mail-in ballots will be coming in through Friday. Note that Republicans returned ballots at a higher rate (36% vs. 27% for Democrats), likely due in part to Democrats holding onto their ballots as the Democratic presidential nomination played out. 

Full Primary Election Tracker