All users who bid on, bought, listed or sold an item within the previous 12-month period. Users may register more than once, and as a result, may have more than one account.
eBay defines this as:
All buyers who successfully closed a transaction on our Marketplace platform within the previous 12-month period. Buyers may register more than once, and as a result, may have more than one account.
Last reported quarter 2020 Q4 it was 185 million, up by 6% year-over-year from 174 million.
Source: eBay Quarterly Results.