Relative-Pronouns – Why is ‘That’ / ‘Which’ Omitted in This Sentence?

That is perfectly OK here. Grammatical number does not enter into the choice between that and which. Both of them (indeed, all English relative pronouns) may be used in both singular and plural contexts.

The only distinction between that and which/who/whom is that that is not permitted in non-restrictive bound relative clauses or as the object of prepositions:

John, that who had prepared better than the rest of us, came in first.
Put it back in the box from that which you took it.

That is rarely found at the head of fused relative clauses today; it is usually replaced by what. , When it is used there it should be understood as a demonstrative rather than a relative, and what follows it is a reduced relative clause with the relativizer deleted.

Your pain is trivial compared to that she suffered = that which she suffered

Some of the prissier stylists demand that only that be employed in restrictive relative clauses, but this has no justification in contemporary or historical practice.

We can generally omit relative pronouns from defining relative clauses if the wh- word does not represent the Subject. The word when replaces temporal Adjuncts, so if the clause is a defining relative clause we can nearly always remove it.

• That was the day when you asked me to marry you.
• That was the day you asked me to marry you.

One case where this does not seem to apply is with relative clauses using the relative word where, although for some strange reason it seems to be marginally acceptable when the antecedent is the word place:

• That's the restaurant where we met last time.
• *That's the restaurant we met last time. (ungrammatical)
• That's the same place we met last time.