The recommended concentration of fluoride in a community water supply is:
Fluoridation is the adjustment of water supply to a fluoride content such that reduction of 50 to 70% in dental caries would occur without damage to teeth or other structures.
In 1958, WHO produced the first report by an expert committee on water fluoridation (TRS 146) and concluded that drinking water containing about 1 ppm fluoride had marked caries - preventive action and that controlled fluoridation of drinking water was a practicable and effective public health measure.

Essentials of preventive and community dentistry
Soben Peter
5th edition
page no. 265
0.1 ppm
0.5 ppm
1.0 ppm
2.0 ppm