Bash Reference Manual
8.2.4 Readline Arguments
You can pass numeric arguments to Readline commands. Sometimes the
argument acts as a repeat count, other times it is the sign of the
argument that is significant. If you pass a negative argument to a
command which normally acts in a forward direction, that command will
act in a backward direction. For example, to kill text back to the
start of the line, you might type `M-- C-k'.
The general way to pass numeric arguments to a command is to type meta
digits before the command. If the first `digit' typed is a minus
sign (`-'), then the sign of the argument will be negative. Once
you have typed one meta digit to get the argument started, you can type
the remainder of the digits, and then the command. For example, to give
the C-d command an argument of 10, you could type `M-1 0 C-d',
which will delete the next ten characters on the input line.