how many of you are there? People in that stage of learning Linux where they google the 5 most common things people get wrong?

Let's be generous and say 1 million. That seems reasonable. It's certainly more than 100,000 and less than 10 million.

Let's be more generous and say that all these people search for this once a week.

At $1 per 1000 pageviews (the generous estimate) that caps income at $1K/week maximum.

That's a nice side-project, if it captures 100% of all the traffic and manage to monetise it (I'd like to see the percentages of Linux users/admins who also install an adblocker; I suspect it's high).

I do this all the time... "I want a service that does X". But I'm not typical, so the market always ends up being small. The standard startup advice of "solve a problem you have yourself" doesn't work for me. I call it my "Kardashian Problem": I don't understand why anyone would ever spend any time paying the Kardashians any attention whatsoever. This is clearly my problem, because the Kardashians are very popular, and the bajillions of people who do pay them attention seem very OK with it. So I am clearly not a very good judge of the market and should not trust my instincts about what will sell well.