In Spanish legal language, "fs." means "fojas" (folio). It is not the same as "page", because it includes both sides, front and reverse.
Black's Law Dictionary gives this explanation and definition:
FOLIO. A leaf. In the ancient lawbooks it was the custom to number the leaves, instead of the pages; hence a folio would include both sides of the leaf, or two pages. The references to these books are made by the number of the folio, the letters "a" and "b" being added to show which of the two pages is intended; thus "Bracton, fol. 100a." (In Spanish, we'd say "a fs. 100" and "a fs. 100 vta." ("vta." standing for "vuelta": reverse)
The preposition used with it is "a": "a fojas XXX de ..."
"autos" is generally translated as "the proceedings", in reference to all the documents that are included and all the steps that are taken in the case at issue. In Black's Law Dictionary, "proceeding" in the singular is defined as the prosecution of a case, but inside the definition and within a footnote we can find some reference to its use in the plural:
[...] Sometimes, merely the record history of a case. [...] The proceedings of a suit embrace all matters that occur in its process judicially.
The phrase in question should thus be translated as: "on folio 9 of the proceedings the spouses enter an appearance..."