What is the number of the USB port is on my new MacBook Air?

See the following:

MacBook Air (13-inch, Early 2015) - Technical Specifications

and

MacBook Air - Technical Specifications - Apple

MacBook Air, M1, 2020, has 2 USB-C ports, that support Thunderbolt 3, and USB 4 (and everything from Thunderbolt 4 apart from dual display output).

Your older MacBook Air from 2015, has USB-A, or USB Type A ports.