There are so many factors I'm not sure anyone can say for sure. It depends on:

1. What your niche is
2. How well the article is written
3. How well the author info box is written to get people to click
4. What offer you have and at what price point
5. How well the offer converts

probably a lot more

The absolute minimum would be one. If you hit the right niche with the right teaser article and got the sig line right, and the offer itself converted, one properly placed article could generate a few sales. I wouldn't quit the day job just yet, though...

Your question is going to be tough to answer, although you'll probably get some specific answers here - most coming from other newbies quoting whatever manual they bought.

There just aren't enough specifics to give you a more concrete answer.

That's a real 'crystal ball' type question really. As Scott says, there are far too many imponderables without seeing one of your articles, how well they are formatted, what the landing page looks like etc etc etc

My goal for every project is to write 90 articles. So far I already have 25 live articles and I'm seeing sporadic sales.

Research your niche first and make sure to know what it is they're REALLY looking for.

Keyword research is very important so stick with it even if it takes you a day to find 2 or 3 relevant keywords that you can rank for.

Then provide a powerful headline, enticing opening paragraph, informative content and a kick-ass resource box.

Good luck