I've just checked "of" on my Mac with HyperCard, and the answer is no - it's not optional. (I tried with "answer the number of cards" and "answer number of cards", which both worked fine, but "answer the number cards" and "answer number cards" both didn't).

If you still have copies of those industrial control HyperCard stacks, I'd love if you were to upload them[1] for the online collection[2] (provided, of course, that doing so doesn't violate any NDAs / etc...).

[1] http://hypercardonline.tk/

[2] https://archive.org/details/hypercardstacks