How to say at most one?

Question

Let's assume that I want to define the concept of injective function:

A function f is said to be injective if each element of the domain has at most one image.

which would become something like that:

函数{かんすう}fによって定義域のおのおののxに対して像が一個以下であるなら、函数fが単射であるという。

But is there something like 少なくとも一個 (at least one) whose meaning would be at most one. By something like 少なくとも I mean that the word order should be the same (ie. (expression whose meaning is “at most”)一個).

Answer 1

In the context of mathematics, 「高々 (or たかだか) 一個」 is the standard expression.

高々 can also be used in non-technical context, but it's somewhat formal. In daily conversations, we'd say "多くて(も)一個" or "最大で一個", etc.