The higher the number the thinner the thread. Also, you have to look to see if the thread is 2 ply or 3 ply.
I use Serger thread on both my longarm and my domestic. I prefer Maxi-lock or Signature.
You can also use So Fine or Bottom Line from Superior threads.
Hope this helps.
Diane Gaudinsky actually prefers 100 Silk thread which is like a piece of fine hair.