Who is Vs Who are?

the first three are correct...

How many people are online?

Thanks, Region.

Can you let me know why the first three are correct but not the last? The sentences are of the same structure, to me.

We use 'who is' either when we know the answer will apply to one person only, e.g. who is in the toilet? or when the number could be one or more, e.g. who is coming to tea?. We use 'who are' when we know the answer is in the plural and it is expressed in the question, e.g. who are the people in this photograph?, who are the culprits?

Expressed better...
We use 'who is' when we are asking about a number of persons that might be one or more, and we use 'who are' when we are asking about definitely more than one.

Thanks, centrox.

What is I hear a few people talking. Can I say "Who are at the door?"

Thanks.

no

Who is at the door, or who are those people [I can hear talking] outside?

Thanks, centrox.