How can I duplicate a file x times in a different directory with terminal?

Just prepend the directory name before the filename(s):

i=0; while ((i++ < 100)); do cp index /destination/directory/"index$i"; done

Replace /destination/directory with the actual directory name.

This assumes the file you want to copy is in the current working directory.
Replace /path/to/destination with the path to the real directory as required.
index should be replaced with the real filename if necessary:

for i in {1..100}; do echo cp -v -- index /path/to/destination/"index-$i"; done

Remove echo after testing, and repeat the command to actually copy the files

It would be better to make the numbers fixed-width for easier sorting, ie 001,002... 010 etc, so you could use printf:

for i in {1..100}; do printf -v new "index-%03d" "$i"; echo cp -v -- "index" /path/to/destination/"$new"; done

or more readably

for i in {1..100}; do
  printf -v new "index-%03d" "$i"
  echo cp -v -- "index" /path/to/destination/"$new"
done