You can't know precisely unfortunately:
1- when decided at client mail (so most of the time) level, then you can imagine than more of 99% of your recipient recieved the HTML version: it's hard today to find an email client that let you decide which email version it will display
2- the only way to have a correct estimation is to count the numbers of clicks in the tracking dedicated to the text version and compare it to the HTML version: there is no "open tracking" method with the text version... And today, even for HTML, open tracking is less and less reliable (iOS15 feature, gmail long mail feature, etc)