Wikipedia and some math. I figured you could find it yourself, if you wanted to.
https://en.m.wikipedia.org/wiki/Visual_acuity
The part about 6/6 (AKA 20/20) vision is the relevant part. The first part is the best possible human vision. Regardless, with those numbers you can do the calculations yourself, if you don't believe me.The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes,[19] this gives an angular resolution of 0.008 degrees, and at a distance of 1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD).
6/6 vision is defined as the ability to resolve two points of light separated by a visual angle of one minute of arc, corresponding to 60 PPD, or about 290–350 pixels per inch for a display on a device held 250 to 300 mm from the eye.[20]
