You can't bulk link tickets using bulk change. If you have the structure add-on you can create the links by dragging and dropping on an active structure (one that makes changes to JIRA tickets) or if you know the keys, type them out in notepad using xxx-1234,xxx-1235,xxx-1236 format and copy/pasting them into the linked issue field on the edited ticket is another possibility.
You may find it easier to link from the story rather than the TC (assuming that the story has many test cases and not vice versa).
Hope this helps,
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