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 2018 Q3 it was 27.08 million, up by 10% year-over-year from 24.70 million.
Source: Yahoo Quarterly Results.