[rcoveson]

No, that's a ZWJ sequence. Those can be arbitrarily long. Doesn't explain where "5 bytes" comes from.

[throwaway2037]

Is there a maximum number of "zero-width joiner" (ZWJ) sequences that can be combined to create a single-width emoji? (Yes, I know the term "single-width" is a loaded term.) I cannot find a precise answer.