At how many users of your web app is it ok to consider this a potential business?

Just one.

With just one user any web app is a potential business. If one person is interested then there are probably hundreds of thousands of people just like them who would be interested as well.

But I don't think your user count matters in the way you're implying. The number of people willing to give you their money for whatever you're selling is the number that matters.

If 0 people want to give you their money for whatever you're selling, and 0 advertisers want exposure to them or their behavioral data then you haven't really built anything of value.

I always think that at 1 user you have a startup, but if you get 1 paying customer you have a business.

The user count really doesn't matter. I have had a product with 4000 monthly active users which still failed !

What matters is how many of these users find your tool indispensable. That could be because you are solving a problem no one else is solving that well, or because you got your users into a habit of using it, so that they are unwilling to try other alternatives.

There are many ways of validating this. If you want to find out BEFORE you build, check out this blog I wrote on the subject - https://medium.com/makelytics/introduction-to-the-problem-canvas-166f47d7597d

Send your users and email add ask if they will pay X/mo for XYZ feature(s). If you don't get any thing back your probably screwed.

Yes, user count does not matter that much, and can be misleading. There are many apps with many users who have near zero intent of paying. Particularly in consumer focused apps or tools.

The test for me is just, are users willing to pay for it? Build a premium product, or pay wall some existing features, and test.