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.