The use of "may only" and "may not" in standardized or mathematical language [closed]
It could mean either one; the correct meaning has to be determined from context.
The meaning of "the resulting set may only have one element" may be either
- that it is possible for the resulting set to have only one element, or
- that it is mandatory for the resulting set to have only one element.
Since the sentence is talking about "the resulting set," it is reasonable to guess that it's expressing a possibility, not a mandate. But more context is needed in order to tell for sure.