When you're new to the game, every purchase is important, every milestone huge. Ten pairs, 20, 100. After that, things tend to get dicey. First you start approximating: "Oh man, I don't know, 300 pairs?" Then, when you're a true vet, you start approximating DOWN, partially so your girl doesn't get mad, and partially because the reality is simply too terrifying to contemplate: "A thousand pairs? Nah, it's only like 500. At most. Seriously."
You keep track of exactly how many pairs you have.