How to have repeated command bash code/script equivalent typing Alt n

If you mean, repeat one character n times, you can do:

printf -v string X%.0s {1..12}

Would store a sequence of 12 Xs in $string.

Or:

$ echo "$(printf X%.0s {1..12})"
XXXXXXXXXXXX

Though that implies forking an extra process.

You could also use a helper function like:

repeat_string() {
  awk -- 'BEGIN{for (i = 0; i < ARGV[1]; i++) printf "%s", ARGV[2]}' "$@"
}

And then:

echo "$(repeat_string 12 X)"

In zsh, you can use its padding operators:

$ echo ${(l[12][X])}
XXXXXXXXXXXX

That's a null expansion left padded to a length of 12 with Xs. There's a similar right padding parameter expansion flag.

For count and character stored in variables:

n=12 c=X
echo ${(pl[$n][$c])}

zsh also has a repeat keyword (inspired from tcsh's):

$ echo "$(repeat 12 printf %s X)"
XXXXXXXXXXXX

With bash, you could always write a repeat function that can repeat simple commands:

repeat() {
  local n="$1"
  shift
  while ((n-- > 0)); do
    "$@"
  done
}

And then echo "$(repeat 12 printf %s X)" like above.

You could emulate a repeat keyword to repeat compound commands with an alias:

alias repeat='i=0; while test "$((i++))" -lt'

And then do echo "$(repeat 12; do printf %s X; done)" for instance.