Is there an industry standard on how many zoom levels should be tested when working on a website?
I know of no "Industry Standard".
And whenever I hear this sort of question, I always think "What industry do you mean?" The testing industry? The software industry? The website industry? The industry that offers an application identical to yours?
For your particular website, what kind of zooming do your users do? That's where you should be concentrating your testing.
I believe in response to a similar question you said "It depends on the requirements or what the stakeholders want." That was good advice. Unless you work in a regulated environment, check with your requirements and stakeholders.