It's because of your use of the g flag.

To quote the documentation:

• If the g flag is used, all results matching the complete regular expression will be returned, but capturing groups will not.
• if the g flag is not used, only the first complete match and its related capturing groups are returned. In this case, the returned item will have additional properties as described below.

You can fix this by simply removing g from your regular expression, then name.match() will return capturing groups as well.

If you only want the number in the brackets, leave out .* after it.

You also don't need any of the flags. i is for case-insensitive matching, but you don't have any letters. m makes ^ and $ match line endings, but you don't have any multi-line strings. And g is for foring all matches, but since the pattern is anchored there can only be one match.

And since the pattern is now the whole match, you don't need a capture group around it.

const arr = [
  'I am not valid',
  '[1] My well-being',
  '[1] I conversation about',
  '[3] I role within',
  '[3] I members in the company',
  '[3]I priorities',
  '[3]How other teams',
  '[4] I within a team',
  '[4] I am a peer',
  '[4] I am a label',
  '[4] I am a label'
]

const regex = /^\[\d+\]/;
for (let index = 0; index < arr.length; index++) {
  const name = arr[index];
  const match = name.match(regex);
  console.log(match)
}