Do I need antivirus software with Catalina 10.15.7

No. I haven't seen anybody who truly had a virus on a Mac in about 20 years. That's not to say there aren't plenty of people who are willing to sell you something. Snake oil vendors. There may be malware, but those aren't viruses, and even then Apple's own system has some security features built in. Mind you, Catalina is no longer being updated so the best thing is to just be careful in what you download. If you want you could scan with a tool such as Malwarebytes but frankly it has never detected anything on my computer and I was running a 12 year old OS until recently.

No: Effective defenses against malware and other threats - Apple Community