Is this an authentic Apple Support phone number?

The number shown is not confirmed as an authentic Apple Support phone number. Apple’s official U.S. support number is 1‑800‑275‑2273.

Any time that you see a phone number in an unsolicited "security" message, or a message about some expensive "purchase" that you do not believe that you really made, assume that it is a scam.

As the FTC says in one of their articles, real security messages will never include a phone number.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

No, but more importantly, would be how you were presented with that number and steps you took after.