I assume you mean the future of the third and fourth conjugations?
No, according to Weiss (2009) it's every identifiable future. The 1st and 2nd conjungation future endings also continue the subjunctive of the root aorist of *bʰuHₓ (> *bʰu̯eti > bit).
There is a concept of "analytical declension" in Slavistics, when declension is expressed using prepositions rather than morphological change of a word. So, it can be used in teaching English to natives of syntactic languages. For example, in the sentence "culture of Poland" (cultura Poloniae), "of Poland" is a genitive case of "Poland".
How many cases are there in Russian according to this line of thinking, counting all the prepositions?
I guess it depends on what you think counts as the subjunctive; must it be a discrete verb form, or can it be a meaning, however expressed? If the latter, don’t all languages then have a subjunctive, even though it may not be a grammatical form, in that all languages can express a wish, irrealis, and so forth?
With my previous reply I tried to make it clear that in linguistics, the subjunctive is not a semantic category ("meaning"), nor cognitive, but a formal one. And that no formal category of the subjunctive has been distinguished in Slavic to my knowledge. The concept of syntactic case is of course used in linguistics, but conflating it with morphological case to say that English has 98 cases is what bloggers used to do on livejournal in the 2000's to blow everyone's minds.
In this thread, aspect was mentioned a few times, as a way of expressing subjunctive in Russian for example.
The Russian aspect does not directly express any of the subjunctive's meanings. Tlepolemus simply called the "hypothetical" ('I would say') use of the subjunctive "the subjunctive", and then said that it requires the perfective aspect. Again, they're confusing form and meaning here and additionally using the heuristic "if it translates with something called X in another language, therefore it's X in every language" beyond reasonable measure. In Russian itself, there's no limitation on which aspect
бы goes with, and neiter are the conditional and modal uses strictly distinguished - they both express the
irrealis modality. Thus the imperfective 'я бы этого не делал' means "I wouldn't do that/be doing that" (Tlepolemus' subjunctive, everyone else's
modal/irrealis expressing an advice) and the perfective 'если бы я этого не сделал, ничего бы не вышло' means "If I hadn't done that, I wouldn't have made it work" (Tlepolemus' conditional). And 'шёл бы ты' means "would you piss off". So even if we wanted to describe Russian grammar using Spanish terminology, Tlepolemus' generalisation would still be incorrect.