I had a message that my iPhone has viruses. I reset all my data. How can I be sure that my phone has no more viruses?

Such messages are scams from criminals. Close the offending page and avoid sites that are involved in such bogus advertising tactics.

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

The message was a scam. Your phone never had a virus to begin with.

There are no known viruses in the wild that can affect an iOS device that has not been jailbroken.

Your phone never had any viruses. The notice is a scam.