In the 37,000 volunteer phone study, over 74 percent of people typed with both thumbs, which was significantly faster than typing with one thumb or an index finger.

In the source's own words

“Over 74% of people used both thumbs for typing which is significantly faster than using one thumb or index finger.”

How do People Type on Mobile Devices? Observations from a Study with 37,000 Volunteers, Kseniia Palin et al., Proceedings of the 21st International Conference on Human-Computer Interaction with Mobile Devices and Services (MobileHCI 2019), 2019, Study summary.