The thread should be thin. Most likely much thinner than you are used to.
This is not a thread for hand-made embroidery (floss) and, moreover, not a thread suitable for darning socks.
You will need a VERY fine thread. Threads are suitable for machine embroidery, or for overlocks. These yarns are labeled “100”, “120/2” or the like. The thread marked “40”, “40/2”, “60”, “80” will definitely not suit you – these are too thick threads.
Good threads in spools of 3000-5000 meters. This spool is enough for 5-10 works.
For your convenience, I made a file:
Download Thread thickness meter (PDF)
You need to print it on A4 sheet without changing the scale. Next, visually compare the thickness of the threads, you need a thread with a thickness of 0.10 – 0.12 mm – these are the thinnest threads.