How do I specify how many "litterae" or "castra" there are?

Question

Certain words in Latin have a special meaning in the plural, which is often translated with the English singular. One obvious example of this is litterae, -arum, which means, "a letter."

Litteras, credo, misit alicui sicario qui Romae noverat neminem. (Cicero, Pro S. Roscio Amerino 76.7)

^{I believe he sent a letter to some assassin who didn't know anyone in Rome.}

Other examples of this kind of word include:

• bigae, -arum: a two-horse chariot
• castra, -orum: a camp
• hibernae, -arum: winter quarters (plural in English too!)
• nūptiae, -arum: a wedding
• quadrīgae, -arum: a four-horse chariot

My question: How do I explicitly refer to a certain amount of these nouns?

Are the following naive translations permissible?

Modo unas litteras tibi misi.

^{I only sent you one letter.}

or:

Duo castra inter flumina posita sunt.

^{Two camps were set up between the rivers.}

Answer 1

You should use a distributive. Cicero, ad Atticum, 5. 3:

ibi mihi tuae litterae binae redditae sunt tertio abs te die

This works for all such plural nouns, but you should take care over the case and gender. As with some larger cardinal numbers, it's a common mistake to forget that.

[Additional clarification]

1. Unae litterae is not necessarily as wrong as I suggested in answer to a comment by @kkm, but the uses of unus and singulus are somewhat subjective. Where unae litterae is found, it's little different from plain litterae; but if you need to write of more than one [camp, say] at different places, you should use castrae singulae [Romae et Ostiae, etc] positae.

2. In English, 'one', 'single', 'only', singular', unique', 'sole' etc. each have their uses, which are sometimes problematic in translating between the two languages. I would translate the example adeo ut una castra iam facta ex binis viderentur from the comments as 'indeed that a single camp would appear (sc. to have been) made from two', but I might equally well write 'just one camp'.

3. A separate question might be better; these are muddy waters. The problems are partly caused by the lack of a plural for the indefinite article in English, and of any indefinite article in Latin.

Answer 2

This doesn't answer your question (Tom Cotton's answer does that), but it's an interesting datum and way too long for a comment.

Here is an (in-)famous sentence from the papal bull Dum Diversas by Pope Nicholas V, issued in 1452. Good luck finding the main verb:

Nos igitur considerantes, quod contra Catholicam fidem insurgentibus, Christianamque Religionem extinguere molientibus, ea virtute, et alia constantia a Christi fidelibus est resistendum, ut fideles ipsi fidei ardore succensi, virtutibusque pro posse succinti detestandum illorum propositum, non solum obice intentionis contraire impediant, si ex oppositione roboris iniquos conatus prohibeant, et Deo cui militant, ipsis assistente perfidorum substernant molimenta, nos, que divino amore communiti, Christianorum charitate invitati, officiique pastoralis astricti debito, et quæ fidei, pro qua Christus Deus noster sanguinem effudit, integritatem, augmentumque respiciunt, nobis fidelium animis vigorem, tuamque Regiam Magestatem in hujusmodi sanctissimo proposito confovere merito cupientes, tibi Sarracenos, et Paganos, aliosque infideles, et Christi inimicos quoscunque, et ubicunque constitutos Regna, Ducatus, Comitatus, Principatus, aliaque Dominia, Terras, Loca, Villas, Castra, et quæcunque alia possessiones, bona mobilia, et immobilia in quibuscumque rebus consistentia, et quocunque nomine censeatur, per eosdem Sarracenos, Paganos, infideles, et Christi inimicos detenta, et possessa, etiam cujuscunque, seu quorumcunque Regis, seu Principis, aut Regum, vel Principum Regna, Ducatus, Comitatus, Principatus, aliaque Dominia, Terræ, Loca, Villæ, Castra, possessiones, et bona hujusmodi fuerint, invadendi, conquerendi, expugnandi, et subjugandi, illorumque personas in perpetuam servitutem redigendi, regna quoque, Ducatus, Comitatus, Principatus, aliaque Dominia, possessiones, et bona hujusmodi, tibi et successoribus tuis Regibus Portugalliæ, perpetuo applicandi, et appropriandi, ac in tuos eorundem successorum usus, et utilitates convertendi plenam, et liberam, auctoritate Apostolica, tenore præsentium concedimus facultatem, eandemque Regiam Magestatem tuam rogamus, requirimus, et hortamur attente, quatenus virtutis gladio præcinctus, ac forti animo præmunitus, pro divini nominis augmento, fideique exaltatione, ac animæ tuæ salute conquirenda, Deum præ oculis habens, in hujusmodi negotio, potentiam virtutis tuæ extendas, ut fides Catholica, per tuam Regiam Magestatem contra inimicos Christi triumphum se reportasse censeat, tuque coronam æternæ gloriæ, pro qua militandum est in terris, quamque promisit Deus diligentibus se, nostramque, et dictæ Sedis benedictionem, et gratiam exinde valeas uberius promereri.

This might be a rare case where English is more concise than Latin, since the sentence is usually translated "You can enslave non-Christians."

The word castra, clearly intended to be understood to refer to camps (plural), appears in many other papal bulls in the same book, in similar contexts. So, I'm thinking that the Latin plural for this word, at least, is understood to mean both one and many.