Yahoo defines this as:
Total of membership IDs of Yahoo! Premium members, Yahoo! BB subscribers, monthly paid-subscribers of digital content and services provided by Yahoo! JAPAN and partner sites* (*IDs with payment through Yahoo! Wallet only). Paid-membership IDs are counted separately for each service, even if there is an overlap in IDs.
Last reported quarter 2019 Q1 it was 27.65 million, up by 6% year-over-year from 26.04 million.
Source: Yahoo Quarterly Results.