Singular or plural form of noun adjective

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And awkward examples do not help. These are appropriate:

The number of teachers needs to be curtailed.
The teacher supply needs to be curtailed.

Thank you.

What I have trouble is when a plural form of a noun is allowed and a singular form of a noun is preferred.
I did a Google Book Search and seemed to have seen both "teachers supply" and "teacher supply" with seemingly no difference in intended meaning.
Sorry to use the words 'teachers" and "teacher" like an object but that wasn't my intent.
I see more singular versi

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'Seemed' to have seen? Could you supply a link to teachers supply that means ' a supply of teachers'?-- at least, that is how I am interpreting your sentences. If they mean "a teachers' supply of chalk", then that is a different matter. Yes, usually the singular noun is used (as adjective) rather than a possessive form, either singular (teacher's) or plural (teachers'

"...usually the singualr noun is used (as adjective)......"

How about "contract manager" vs "contracts manager"?

Is a contract manager "a manager employed on a contract basis (not permanent) regardless of their job scope" while a contracts manager is employed to handle contracts, or they both could be used interchangeably to mean a manager whose job is to handle con

The singular could be either. There are uncommon cases of plural-noun-as-modifier, and often in such cases where confusion might occur.