The shade of lens you select depends on multiple things. It depends mainly on the Amperage you are using, the material on which you are working, and finally, brightness perceived by your eyes. All these factors contribute to the brightness produced by welding. Higher the brightness, darker the shades to be used. Usually, lower Amperage is used for Tig welding; hence shade 10 is often used. To Understand better, have a look on practical figures below to get better understanding.
For tig welding on mild steel, Amperage would be 5 to 10A, and preferable shade would be 9
However, for 20 to 30 amperage 10 shade would be suitable.
An important conclusion to take, while selecting a particular helmet, you should investigate available shades welding helmet can offer. Whether those shade fit your criteria of work or not.