Bing Search API pages state:
Each page of results returned for a query uses a single transaction and will count toward your transaction limit.
and:
A transaction is one request that returns one page of results. Retrieving multiple pages will result in multiple transactions executed.
I am unable to find any place where it tells how pages are calculated. Is it amount of results returned (what if I ask for top 10) or it depends on sources requested (web+image+video+news+spell)? Anyone working on it or knowledge on it, please provide some
details.