@Suttilak-Wattanapaiboonsuk There's an autocomplete when you write a tag.
What I've done on my forum is that I created a subject in an hidden category where I create each tag I want to appear in ACP (to color them) because tags with 0 uses are still visible and themable. You are limited to 5 tags by default but you can change them just need to submit post to keep the tags 😉