Need is a strong word.
If you have your computer in your basement, not connected to the Internet, that you use as a glorified typewriter, then maybe you don't need Anti-Virus, because nothing is downloaded.
Maybe you like to download things from the Internet, and try new software. In this case, you'd be safest running Anti-Virus.
If you are running a service where you accept files from others, then you most certainly need to run Anti-Virus, even if the viruses don't affect your system, as to prevent yourself from propagating the virus and being irresponsible.
Without further details, it will be hard for anybody to give you an accurate answer. Your use-case and practices will impact the advice given.
clamav is often used on Linux systems to scan files for malicious content, for various purposes. Unless you're trying to use some ancient hardware, anything should be capable of running good Anti-Virus with minimum impact.
Anti-Virus options haven't changed much with Ubuntu 18.04. This question (and its duplicates) are still good sources of information.
Anybody telling you that you don't need Anti-Virus is probably giving bad advice, or has a lot of knowledge about Linux systems. Again, your practices, and knowledge, play a big part in your needs.