You keep track of exactly how many pairs you have.
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."