You can advertise without any visitors. I do, and it doesnt affect people coming to the site.

There is no magic number

Normally around 1k uniques a day you can start to generate interest, depending on content of the site, once you get above 2k-3k month you can get places like tribalfusion to be interested or fastclick (I think that is the name)

Text links sell on sites with low traffic and high PR but they generally do not go for much.

I would suggest to get atleast 50 to 75K views of your site before starting advertising it.

Yes, some of the larger ad agencies / networks are Tribal Fusion, ValueClick, FastClick, Burst! Media, etc. Don't underestimate the power of text links. They can be very valuable in getting your site well indexed by the search engines and consequently can run into the thousands per month for a run of site link on a high profile site.

Uniques dont really matter much when you are advertising its the page views that matter the most. There is no set figure either, depends on the niche, if you are a small site but only site in a niche then you can get good advertisers very quickly.

I would suggest to get SEO of your site and target it to your niche market.

well it depends on what type of advertising ypu will be selling.

You cans start selling textlinks when you get a google pagerank of 3. You will be able to sell a link for about $5 per month on average (depending on your website).

If you want to start selling banners you will need to get at least 500 unique visitors per day to make it worth while for the purcheser.

But this all depends on your websites target market.

Hope this helps with your question.

what is your site's orientation?