Looking for an API that can provide the amount of flights bought from point a to point b on daily basis
Not sure you can get the data easily.
IATA developed the NDC standard an API for airlines, which might enable you to check flight bookings between two destinations.
Nonetheless, as with any industry wide standard, it is interpreted in many different ways by the companies implementing it. Hence a pain to develop.

The most simple solution would be to use a set of well defined APIs from an airline ticketing company. Amadeus is one of them and I am almost sure they provide these kind of information.

In both cases, you are looking at quite a high upfront cost (in the thousands).