Kate would be title "Queen Catherine II" because their has already been a Catherine?

No. Kate would be a Queen Consort (deriving title and status from William).

There have been queens named Catherine, but none have been a Queen Regnant (ruling on her own). All have been consorts.

There’s never been a Catherine, at least not one that gets a number, and Kate won’t get a number either.

No. She will only be a Queen Consort, she won't have a number. William will be William V.

NO as is NOT going to be the Monarch..Just the wife of a King so does not get a number...

No, because consorts don’t get numbers and there has been five of them thus far in the UK and it’s predecessor kingdoms

Henry V's Consort

Catherine of Valois

Henry VIII's consorts

Catherine of Aragon

Catherine Howard (used the variant Katherine)

Catherine Parr

Charles II's Consort

Catherine of Braganza

There have already been two!

No. Catherine will be a consort, not a reigning monarch. It is only a queen regnant who has the ordinal number after her name. Catherine will be known by any variation of"Queen Catherine," "Catherine, Queen Consort," or "Queen Catherine, the Queen Consort."

And, in the UK, there have been no Queens Regnant named Catherine!

No, she will have no number to her name, she is just william's queen consort, when he is king the titles will be william V and queen catherine.

who is Kate

Her title will be Queen Consort