Say I have a non-fiction blog that generate an amount of frequent readers. A publisher tells me that I can only publish when I can sell 2000 copies. How can I measure my traffic to know whether it's publishable or not?
Traffic by itself is not enough. I can have 1 million viewers every month on my website, but none of them will interact with my content. Does that mean I have "enough traffic to sell 2000 copies"?
You need an extra step to convert these people from strangers on the internet to X be it members of your blog, subscribers on an email list, or whatever makes them more invested in your creation. Find a way to make them feel you're more than a google search result.
The usual way to do that is to ask for them to subscribe or register and offer them exclusive value in return. Secret course? e-book about something that interests them? A video they want to see that isn't available to the public?
Now, if you have an email list, always assume that no more than 20% of people will even open any of your emails. From those people, always assume that maybe up to 20% will buy your stuff (80/20 rule) Now you just need to calculate how many subscribers you need so that your 2000 buyers are your 20%.
The minimum of that is:
2000*5 = 10000 that's the number of people opening your emails.
10000*5 = 50000 that's the minimum number of people you need in your email list to make 2000 sales to that specific email list.
I'm sorry if it seems grim. If it makes you feel any better, I'm on the same boat on my way there :)