The highest code point of UTF-8

UTF-8 is capable of encoding all 1,112,064 valid Unicode code points using one to four one-byte (8-bit) code units.

The maximum valid code point in Unicode is U+10FFFF, which makes it a 21-bit code set (but not all 21-bit integers are valid Unicode code points; specifically the values from 0x110000 to 0x1FFFFF are not valid Unicode code points).

mb_chr outputs nothing if you input any number greater than 0x10FFFF.