Unicode code points go up to 0x10FFFF, which fits in 21 bits. If this were ever to become a problem (which I don't see happening any time soon), the transition

Unicode code points go up to 0x10FFFF, which fits in 21 bits. If this were ever to become a problem (which I don't see happening any time soon), the transition from UCS-2 to UTF-16 is prior art on how to pull off the extension of a coding space. Somewhat unrelated, but nevertheless worth mentioning as it's a common misconception: While UTF-32 is a fixed-length coding for Unicode characters, often, the more interesting unit is the grapheme cluster, effectively making UTF-32 into a variable-length coding.