We know that if a smartphone has more than 300 pixels per inch, it is a retina display. How much ppi it need to be a super retina display?

It is said that human retina can distinguish pixels and letters in screen seem broken if ppi is below 300. If ppi is above 300, how high the ppi may be, it looks same for human eye, then why iPhone has 458 pixels in iPhone x?

For the record Retina display & super Retina display are just marketing terms. As such there's no threshold for either.

A "super retina" display is just a marketing term, it just means that it has more ppi than a standard retina display. Hope this helps