The chart above uses the official figures from the office of the Texas Secretary of State. It shows the total number of early voters in 15 of the largest counties in Texas. These counties contain 63.15% of all the registered voters in the state.
Note that the early voting totals for 2020 are far ahead of 2016. About 759,638 Democrats voted in 2020, while only 448,859 voted in 2016. I think it's pretty obvious that the total number of primary voters in 2020 will exceed that of 2016 after the March 3rd voters are figured in.
The only question is whether those voters will exceed the number of Democratic primary voters that voted in 2008, when a record number of voters participated (2,874,986). The chart below shows the total number of voters in the past four Texas Democratic primaries.