I have built a site and am convinced that it can get huge numbers and traffic based on the value proposition. I think journalists will write about it and people will want to check it out. It is free to use. The plan is to get the number( if my confidence is not unjustified & not just a pipe dream) and then decide on the best way to monetize it based on where users find value and how they use it. We obviously have ideas about the various ways it could be monetized. Is it being daft not to have this plan set in stone from the outset? Is it naive to believe numbers can always be turned into revenue, (ie value can always be monetized). Costs of running the site are fairly low (other than hosting, support etc)
The DIY startup method - build an audience around an area of your expertise, deliver that audience content they care about, then engage them to see what product of yours they would be willing to buy. Tim Ferriss (FHWW), John Lee Dumas (EOF), Bryan Harris (10KSubs), and countless others have taken this approach and turned out scalable businesses. It's great to have an idea of what visitors may want to buy, but the minute you build product before you build an audience for it, you immediately risk taking the wrong path at the expense of time, money, and momentum.
Even without knowing your industry, how you engage your visitors should be the biggest piece here - it's great to bring a bunch of people to one place, but if you aren't getting their contact info and making them stick around with content, discussions, videos, emails then it's going to be much harder to bring them back, let alone get them to buy from you. This can be as simple as blog posts, as timely as real-time video webinars, or as easy as delivering curated relevant articles via email. The end result should be you as founders and/or your site being seen as the best place to get information your audience cares about.
If costs to you are low (negligible, or easily offset by adding a small revenue stream), and you have the ability to deliver a knowledge base to your audience around your expertise, then go for it! The right level of engagement with that audience should give you some great insight into what they would buy, how much they would pay for it, and how likely they would be to share their experience with others. It can take a few shots to find the right balance of product vs. demand, but having that knowledge base and content allows you to be flexible in your approach. Don't assume you know what your visitors would buy - test your assumptions by simply asking them. Get an understanding of the biggest questions they want answered, answer them through content, and use this insight to build product. There is no 'right number' of visitors - even getting 100 email addresses can be a huge start to understanding what to build.