What anti virus should I be running?

The very best antivirus, cleaning and overall maintenance app for Mac OS is Mac OS itself. If simply kept up-to-date and otherwise left alone Mac OS runs securely for most users for many years. I have never had any antivirus, cleaning apps or other "maintenance" apps installed and for over 15 years of Mac usage I have NEVER had any problems.

Absolutely none!!! Mac's take care of themselves. Keep your operating system up to date and you'll be fine. Most of the "anti virus" programs advertised to protect your Mac and speed it up only mess things up and slow it down. I've had Mac's since 2005 and only had one trojan hit that rode on a reader update. It placed multiple adds on every web site I visited. My Mac was long out of warranty but when I called Apple support, they helped me get rid of it in 10 minutes...no charge.

None are needed, because Mac's do not get PC viruses.

You can however get Malware and Adware, if you use third party browsers or download crapware from un-approved sites.

for that see > https://www.thesafemac.com/mmg/

Personally I have never run any. See: macOS - Security - Apple

None. There are no Mac viruses. There is no AV software - none - that will stop you from installing a Trojan. They are a complete and utter waste of system resources.

You can however get Malware and Adware…

A virus is malware. As is a Trojan or worm.