that is/are

I would like to know if after ‘that’ we use a singular or a plural verb.For instance:

e.g. That’s the cups I bought yesterday.
That are the cups I bought yesterday.

That’s 2 sixes in a row.
That are 2 sixes in a row.

That is the cup…
Those are the cups…

That is two sixes in a row.

Why the latter is in the singular form and not in the plural one?

Because it is one row of two sixes.

The row is singular.

Could I use in these sentences the singular form:

There was 1 million of reasons.
That’s 2 cups.

“1 million of reasons” is wrong. It should be “1 million reasons”, or, more usually in this kind of context, “a million reasons”. “millions of reasons” would also be possible.

“There was a million reasons” is not good English. You should say “There were a million reasons”.

Informally, “That’s two cups” may be tolerable in certain contexts if the cups are treated together as one item. (Generally, the language is more tolerant of the use of “that’s” with plural subjects than is the case with “there was”.)

“That’s two cups” is also OK in the special case when “two cups” describes a quantity, as in cooking (cf. “five hours is a long time”, “five miles is about eight kilometres”).

But ‘a million reasons’ doesn’t describe also a quantity used with the singular ?

‘a million’ is not singular. ‘one’ (unit) is singular.

There is one reason.
There are a million reasons.

There is one row of six.
There are a million rows of six.
There is one row of a million sixes.
There are a million rows of a million sixes.

There are a million paperclips.
There is one box of a million paperclips.
There are many boxes of a million paperclips.

The difference is not so much between “two cups” and “a million reasons” as between “that’s” and “there was”. Certain forms, such as “that’s” and “there’s”, are widely tolerated with plural subjects in informal English. So, in conversation you could say “that’s three reasons for a start” or “there’s a million reasons why you shouldn’t do that”. However “there was” does not sound good with plural subjects in standard English (though you may hear it in certain dialects and in uneducated speech).

Thank you for replying.