So I am wondering how many visitors I would need to my website to start charging for adverts. Any advice is welcomed. I am guessing there is no real formula as such.

Local, regional, national or international advertisers?

Mind you this is just one question of probably hundreds that need answering prior to even a good guess.

And make no mistake there absolutely is a formula.

I am thinking international (e.g. magic suppliers).

Charging for ad space is about potential click-through rates and not overall traffic.

How many click-throughs can you provide?

Our newsletter gets 12% click-through from 8200 active subscribers. So when we run an ad, they get just shy of 1k click-throughs to their offer.
Because the list is all buyers, it's a higher value click, so we can charge a much higher fee than website adverts.

But it is still based on click-through...
If your ads generate 3% click-through and you have 10k visits a month, you can tell them an estimated traffic.

Use an ad manager that gives you analytics on clicks so you can automate reporting.