cara is one side (recto/verso - verso/anverso in some countries) of a sheet (folio). In Asker's context, there may be a fixed charge for n 'sides' and a per-side surcharge for the 'excess'.

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Note added at 2 hrs (2023-02-12 14:50:18 GMT)
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@Phil
In the variants of English I am familiar with, we would say, for example, 'legal documents are usually written on both sides (caras) of the paper'. We don't say 'written on both pages of the paper'.

Further explanation:
In the Spanish-speaking jurisdictions I am familiar with (Spain is not on my list) if a notary is certifying a copy, she will usually put a stamp on any blank caras as proof that the empty 'side' is genuinely text-free and has not been skipped during the copying, or removed deliberately. The stamp will say something like Cara inutilizada --> (+/-) 'This side unused'.

Bear in mind also that in legal documents, the folios (sheets) are numbered sequentially, and each 'side' is marked 'recto' or 'verso' (or equivalent terms mentioned above) as appropriate (often omitted on the recto, where each sequential folio number first appears).

In both Spanish and English, the terms página and 'page' are used more loosely, sometimes referring to a sheet, and sometimes to one side of a sheet, and that's why they are usually avoided in notarized documents where more precise referencing is needed.