Orberg’s (slightly adapted) Livy in in LLPSI Cap XLIV is describing the division of Romans into various orders of property and consequent military obligation.
Quinta classis aucta: centuriae triginta factae; his additi sunt cornicines tubicinesque in duas centurias distributi. Undecim milibus haec classis censebatur. Hoc minor census reliquam multitudinem habuit; inde una centuria facta est immunis militia. Ita pedestri exercitu distributo, ex primoribus civitatis duodeviginti equitum centurias fecit.
Undecim milibus haec classis censebatur would appear to be telling us that this particular class comprised 11,000 soldiers. But I feel like I have missed a basic step somewhere. Is milibus a dative? I can’t figure out what case this is (dative or ablative), nor what it’s doing in this sentence.