I have a small hobby web site that acts as a friend finder for folks with similar hobby interests.  So far I have avoided advertising of any kind to keep from having a negative impact.  But recently my user list has started growing nicely.My goal is to pay for the site's hosting costs and other small costs that went into site development:  domain registration, html template and some graphics.  None of it was very expensive.  Hosting is the main cost.How many registered users or regular user traffic should the site have before some simple advertising.  I'm thinking no more than a single ad in any page.
How many users should a site have before using advertising?
advertising;cost
There is no minimum traffic needed for ads.   I have ads on my websites and I have for the past 10 years.   I now make my living from those advertisements.  My traffic started out small, paying for the hosting and keeping the site alive.  You talk of negative effects, but there are not many.   Users that really don't like ads use ad blockers and don't see the ads.  Other users don't seem to mind.  When I first started putting ads on, I expected to see lots of complaints.  Those complaints never materialized.  Google is now penalizing sites that obscure the content with ads, or otherwise use too many ads.  One ad on a page is certainly not going to trigger this penalty.My guidelines for ads would be:Use no more than three per page, and only one above the fold.Don't put ads around the edges of the page.   Put the ad in between content where it will be noticed by users.   Ads above all the content are summarily ignored.   Ads in the right sidebar are not effective.Be careful of ads that mimic site functionality.   Stay away from link bar ads that mimic navigation and advertisers that try to match their ads to buttons on your site such as download buttons.Try different layouts to figure out where ads get noticed the most.