There is going to be ... / There are going to be ...

(a) There is/There's going to be 50 people at the party.
(b) There are/There're going to be 50 people at the party.

I think (b) is grammatically correct.
How about (a)? Is it acceptable?

B is grammatically correct. Most Americans would use the "there's" version of A in spoken English. The "there is" version is incorrect.

[EDITED]

GoesStation said:

The "there is" version is incorrect.

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I somehow typed "are" when I meant "is". I've corrected it now. For B, you may sometimes hear someone say there're, but it's not very common. It's too hard to say.

GoesStation said:

I somehow typed "are" when I meant "is". I've corrected it now.

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It only shows that you're used to there's so much that you subconsciously don't even consider there are an option. ;-)

Glizdka said:

It only shows that you're used to there's so much that you subconsciously don't even consider there are an option. ;-)

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You took the words out of my mouth. I really was thinking of writing that.