Read ad revenue earners talk about UV (unique visitor) and PV (Page Views).
Roughly how many UV , Page Views per mth is necessary to make at least $2K revenue per mth? Let us say on low cost high volume niche like entertainment

While the question is specific enough, there is no specific answer just because the potential variants of such scenario are huge! There is the entertainment content niche, banner design, banner positioning, and, something you cannot control - advertisers' bid amounts on keywords/ads displayed on your site.

You should really approach this problem backwards. Find out what the average click-through rate is for the niche you are looking for, try to do a search for websites that were for sale (to try to get their revenue numbers) and see what banner positions of google ads are on their site, or simply try to understand what clients bid for keywords of such a site, then put two together to get to traffic data.

Usually the more the better because every single visitor could also be a unique potential customer. The leading companies are even counting the clicks made on their web graphics.

have a look around at ad merchant websites, these will give you stats and prices on how much they want for ads.