Unfortunately, this question does not have a single, acceptable answer (12,453 visitors per month, right?) because it varies so much on the age of the site, the quantity of content on the site, and the competition for long tail keywords within the chosen niche.
That said, there are a few different ways you can think about estimating traffic numbers.
Views per Post
The first metric you might use to estimate traffic is to look at sessions (or hits) per post. I generally prefer looking at sessions over total page views, because that gives me an idea of how many people were actually on the site in a given period of time (which is more relevant when thinking about affiliate conversions), whereas page views just shows me how many pages were viewed (which is more relevant for traffic flow data and adsense numbers), both are valid metrics, but that’s the subject of a separate post.
If you’re trying to get a raw traffic estimate, however, page views per post is a good metric to use. Kyle from Wealthy Affiliate claims that you can expect an average of 5 views per day per post. While this doesn’t sound like a lot, the math adds up quickly. Using that as a base you can expect 15, 000 sessions per month for a 100 page site. For a 500 page site you might expect 75,000 hits per month!
Personally, I find that this number feels a little high, and I prefer to use a 2-3 views per post per day metric. Part of that number is a personal preference for being more conservative with my traffic estimates during the early stages of a new site. 2 views per post per day is still a significant amount of traffic: 6,000 views per month for a 100 page site and 30,000 views per month for a 500 page site!
Exactly which number you use is up to you, and you can play with different figures to see what several different scenarios might look like.
I’d much rather assume I’ll have lower traffic, and then be pleased to exceed my goal, as opposed to the other way around.
Not All Posts Are Created Equal
One feature of using a views per post logic for anticipating traffic is that not all posts are created equal. You’ll probably find that once your site is sufficiently developed (after the 6 month milestone, perhaps) that the search engines will favor some posts over others. There are a number of factors that affect these rankings, so don’t worry about it too much, just use the data to help increase conversion for those more popular pages.