"Any requirement" or "any requirements"
Which of the following expressions is grammatically correct:
I will let you know if I come across any requirement.
I will let you know if I come across any requirements.
Either should be syntactically correct, but if you anticipate that there may only be one requirement (e.g., "The class for your child didn't specify a minimum age, but I will let you know if I come across any requirement.") the singular would be preferred. Whereas if it's open-ended and could be any number of unknown requirements, the plural would be the way to go.
Both of the following are grammatically correct:
• I will let you know if I come across any requirement.
• I will let you know if I come across any requirements.
(See comment below regarding quotation marks.)
Each has a context in which it makes sense; as Ben notes, singular vs plural is one determiner. If the sentence is a reply, the form of the question asked is important. For example:
• Q: Do you know if it has to be more than 5? A: I will let you know if I come across any requirement like that.
• Q: Do you know if it has to be more than 5 or less than 7? A: I will let you know if I come across any requirements like that.
Note regarding quotation marks: Every sentence of the following form is grammatical, no matter what letters and spaces you substitute for XYZ. Placing XYZ in quotation marks indicates that it is a literal quotation, so it doesn't affect grammaticality of the sentence as a whole.
I will let you know if I come across "XYZ".