Suppose I want to type '222222' in emacs, how do I use the numeric prefix idiom to achieve this? A naive guess would be 'C-u 6 2' but emacs interprets this as a numeric prefix of 62 and waits for the next command.
C-u 6 C-u 2
universal-argument (emphasis mine):
Begin a numeric argument for the following command.
Digits or minus sign following C-u make up the numeric argument.
C-u following the digits or minus sign ends the argument.
C-u without digits or minus sign provides 4 as argument.
Repeating C-u without digits or minus sign
multiplies the argument by 4 each time.
For some commands, just C-u by itself serves as a flag
which is different in effect from any particular numeric argument.
These commands include C-SPC and M-x start-kbd-macro.
You can find this documentation by typing
C-h k C-u or
M-x describe-function RET universal-argument RET.
Your question title doesn't match the body text?
If you want to "repeat a number" (singular), nanny's answer has you covered.
If (as per the title) you want to "repeat a list of numbers", I would use a keyboard macro. e.g.: