I'd strongly speculate that the rows are 80 characters each.
The original Spanish is as follows:
A cada uno de los muros de cada hexágono corresponden cinco anaqueles; cada
anaquel encierra treinta y dos libros de formato uniforme; cada libro es de cuatrocientas diez páginas; cada página, de cuarenta renglones; cada renglón, de unas ochenta letras de color negro.
Notably, the primary cause for doubt (the phrasing of some 80 letters) is still there in Spanish, and is actually a little stronger in its conveying the idea of an approximation. "Some" in English doesn't necessarily mean there's deviation; it can just be a color word.
But what in English is written as "some 80 letters which are black in color" is written in Spanish as "de unas ochenta letras de color negro." The key word in Spanish is unas - it's not actually strictly necessary, and adds a little bit of ambiguity, because here, it just means "approximately" or "about." It can still be a color word in Spanish, but I think that's less likely. If Borges wanted to say that, yes, they are exactly 80 letters, Borges would probably have left it at "de ochenta letras."
...but, just before that, Borges does say "...libros de formato uniforme," and enumerates exactly the ways that the books in "uniform format." The linking of the books' uniform format and eighty-letter rows in the same sentence heavily implies that all the rows in all the books are 80 characters long.
If you want to doubt that there are 80 characters in a row, there's definitely a reason to, but I don't think it's a very good one.