Number has nothing to do with it. When the verb asserts something that
is not real or true but rather exists only as an idea or imagining, the
verb needs to be in the subjunctive mood. For most verbs, the only
marker of the subjunctive is in the third person, where the -s normal in
the indicative mood is omitted:
If he fail in this, we are lost.
But the verb "be" has a whole set of subjunctive markers. In the third
person, it uses were:
If I were king, things would be ordered quite differently.
Nowadays, with language use becoming ever sloppier, use of the indicative
where the subjunctive is needed are increasingly common, but even in this
age there are some forms where failure to use the subjunctive clangs on
even the tin ear.