That totally depends on what angular resolution you require and what distance you are comfortable with. Using that list Apple's Retina concept starts at 57 and

That totally depends on what angular resolution you require and what distance you are comfortable with. Using that list Apple's Retina concept starts at 57 and 4K 27" is ~60. And even that is assuming you place a 27" screen the same distance away from your eyes as a 24" or even 21" screen as all the iMacs use the same viewing distance. But I'm glad we got over the idea that 220dpi is a magical number. Otherwise not even the iMac or the Apple Pro XDR displays counted as HiDPI.