Is there any authoritative source from where we can find out the number of commercial jets that a country or airline purchases annually?
planespotters.net lists, for many commercial airlines, all their airplanes along with their delivery dates.
As for an authoritative source, you would have to look at the IATA or any other paying site such as airfleets.net.
Wikipedia lists are nice, but think about this question backwards to get the authoritative source. Find and parse the manufacturer data on the exact number of airliners sold.
- Airbus (or direct link to the spreadsheet download on that page)
- Boeing (lots of nice filtering options!)
You could search for older reports or archives of previous years.