Of the four examples, the second seems the most correct.
It will expect that the client has 1 page.
If you don't use that then some rephrasing would commonly be used.
It will expect the client to have 1 page.
If you use a 1 page, you should add something afterwards (a 1-page report).
Alternatively, if you don't add anything after page, you can add a but drop 1 (a page) or you can replace 1 with single (a single page).
The use of it is strange in this isolated sentence, but, in context, might make sense. It reminds me of Golum's style of speech from Lord of the Rings. Or, perhaps, it could be a non-gender-identifying person's pronoun of choice. (Or a conscious machine or object in a speculative story.)
Lastly, it would look better in this case to write out one rather than to to use the number.