"definiert" sounds odd here. Besser wäre: "Der händler legt die Anzahl fest." In German "definieren" is used for the definition of meanings only (technical or legal terms, or relating to programming languages or math).
So the same thing as in English. I love all the similarities with English. The small nuances were far more difficult when learning Spanish.
I wouldn't say that it's the same as English because "to define" can be used in other contexts. For example: "He didn't let the mistake define his career"
Why won't it accept "figure" for Anzahl?