I got a message that I have 12 viruses on my iphone that I had to install an application is how I can know if it is true

All such messages are scams from criminals. Do not interact with the message or the sender. Never download anything based on a popup message.

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

You do not have any viruses on your iPhone. Those are scam notifications attempting to get your to pay for a useless application. Just shut down the web browser and that is all you need to do.

Axel F.