Fluoride in drinking water has no effect on IQ or brain function, long-term study shows
(twin-cities.umn.edu)
According to Wikipedia (I know, not the most accurate source. I just needed a quick statistic) the U.S. has a fluoride level of 0.7 mg / L since 2015. Poking around for other countries, it looks like 0.7-1.2 mg / L is the range that most countries add to their water supply.
For reference, the WHO recommends 1.5 mg / L as the upper limit. Additionally, in many places, the groundwater has fluoride levels a bit higher than that.