All the answers so far are (in my opinion) correct, but the OP’s confusion results from a difference between what an adjectival phrase can accomplish in English, and what a copula can. You can say:

It is a big tent.

or

This tent is big.

but while it is perfectly correct to say:

It is a five-person tent.

it is wrong, or at least heavily discouraged, to say:

This tent is five-person.

Therefore asking “how many person [or persons or people] is this tent?” is not done by native speakers.

Ditto with cores of a processor. In the case of stories of a building, you absolutely can say:

This building is five stories.

because a building consists almost entirely of its stories. Although it may also have basements and subbasements, parking, curtilage, and so on, those are not considered essential to its “buildingness”. There are buildings without basements, but there are no zero-story buildings.

Of course a tent must hold at least one person and a CPU needs a core, but it also must have other things (a roof, a ALU), so we say they “have” rather than “are” these things.

All the answers so far are (in my opinion) correct, but the OP’s confusion results from a difference between what an adjectival phrase can accomplish in English, and what a copula can. You can say:

It is a big tent.

or

This tent is big.

but while it is perfectly correct to say:

It is a five-person tent.

it is wrong, or at least heavily discouraged, to say:

This tent is five-person.

Therefore asking “how many person [or persons or people] is this tent?” is not done by native speakers.

Ditto with cores of a processor. In the case of stories of a building, you absolutely can say:

This building is five stories.

because a building consists almost entirely of its stories. Although it may also have basements and subbasements, parking, curtilage, and so on, those are not considered essential to its “buildingness”. There are buildings without basements, but there are no zero-story buildings.

Of course a tent must hold at least one person and a CPU needs a core, but it also must have other things (a roof, an ALU), so we say they “have” rather than “are” these things.