I imagine you mean - predominantly - in cleft sentences that have 'What' as the subject:
'What I want is a nice hot bath.'
No problem there. But when the thing wanted is plural, views differ. Swan says
Practical English Usage, 130,1.A what-clause is normally considered to be singular; if it begins a cleft sentence it is followed by is/was. But a plural verb is sometimes possible before a plural noun in an informal style. [My emphasis.]
So use the singular (which some people prefer anyway - me, for example ) in an exam. What I do, when there is a plural complement, is invert the cleft sentence: 'More people like that are what we need.' (This doesn't solve the problem, but it 'papers over the cracks'. )
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