How do I use Safe Boot to check for viruses on my Mac?

If you get a message about 5 viruses this is most likely a scam message, Macs are not prone to viruses

If you're worried about viruses, you might want to scan your Mac with the free version of malwarebytes to see if that program can find anything

Booting in the safe mode will not help with these kinds of issues, but if you want the instructions for booting in the safe mode, please see this Link

Start up your Mac in safe mode - Apple Support

There are no known viruses in the wild that can infect macOS. The message is a scam.

Apparently Malwarebytes no longer offers its free version. However, I checked and found a couple of other apps which say they do the same thing and they are free.