hi do i need antivirus software on my mac air

no.

Simple put,  Apple attempts to provide all the malware detection and removal you need in Mac OS X.

"Effective defenses against malware and other threats" by John Galt

Effective defenses against malware and ot… - Apple Community

Phony "tech support" / "ransomware" popups and web pages

Phony "tech support" / "ransomware" popup… - Apple Community

"Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams"

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

"MalwareBytes Anti-Malware for Mac Removes adware and malware Revives your Mac."  MalwareBytes has a more restrictive filter for adware than Apple. MalwareBytes has come to be accepted as the only malware detector you should consider.  For those pestered by browser attacks consider MalwareBytes.

Mac Antivirus | Malwarebytes Antivirus for Mac | Malwarebytes

etrecheck

Run etrecheck.   The first five runs are free. Provided a report on your machines hardware and software.  Great for diagnosing your system.  Click on the download link at the bottom of the screen.

EtreCheck

Apple on Security:

Apple Platform Security - Apple Support

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS. The Built-in Security of Big Sur is all that is required.