Clauses do not have inherent number, but when a clause is employed under the category of an NP^† it must be treated as either singular or plural. A single clause so employed is treated as a singular entity; two or more coordinated clauses so employed are treated as a plural entity.

That John is hungry is an effect of his fast.
That John is hungry, that he is feeble and that he is incoherent are effects of his fast.

_{^†An 'NP' is not the same thing as a 'noun phrase'. A noun phrase is a phrase headed by a noun, while an 'NP' (pronounced En-Pee) is a phrase which plays certain (not all) syntactic roles typical of a noun or noun phrase. Thus a clause (which is headed by a verb) cannot be a noun phrase, but it can be (as in these examples) an NP.}