Learn Portuguese (Portugal): Para quem é o bolo?

In English, we often leave prepositions like 'for' at the very end of a question. In Portuguese, this is impossible! Prepositions like para (for), de (of/from), or com (with) must always stay right beside the question word they belong to. You have to phrase it literally as 'For whom is the cake?'.

Why do we use é (from the verb ser) here instead of está (from estar)?

We use ser because the cake's intended recipient is considered a fixed fact or the fundamental purpose of the gift. You always use ser when talking about identity, origin, and destination (who something is meant for).

Could I use por instead of para and ask Por quem é o bolo?

That would completely change the meaning! Para quem means 'for whom' (asking who gets the cake). If you asked Por quem é o bolo?, you would be asking 'By whom is the cake?' (asking who baked or brought it).

If I know the cake is for multiple people, does quem become plural?

No, quem (who/whom) is invariable, meaning its spelling never changes. You use quem whether you are asking about one person or a whole group of people.