New lore:
In the world this counter is from, humanity has gained the technology needed to add and remove limbs. Oddly enough, it is painless.
Not all use of this technology is legal and voluntary, making this counter needed.
Some individuals are "weird" and like to take breaks from having arms. Such individuals regularly schedule two appointments, one to lose arms, and one to gain arms.
On a similar note, telekinesis was invented as a technology to make up for missing arms. This helps avoid exorbitant costs to get new arms.