Given a flight number, how can one know the maximum capacity (mAh or Wh) and number of external batteries one may have in one's carry-on luggage, without having to contact the airline company or search through their websites?
I think a general answer is not possible. What you can take aboard as carry-on will depend upon the airline's rules, how those rules are enforced (or not) by the airline, as well as the rules and actions of airport security at that particular airport, that particular time, and those particular security workers.
Without talking to the airline directly, you're left with the airline's web statements, whatever information you can discover about or from the security processes at that airport, and prior travelers' reports and comments.
Unless the airline says otherwise (for instance, on their website, boarding passes, tickets...), it is most likely they apply the IATA guidelines.
Power banks are considered as spare batteries: you can carry up to 20 with a capacity of less than 100 Wh each in your carry-on luggage. Beyond that it becomes more complex.