Declension, as far as I know, corresponds to the act of creating boxes where you can pile up nouns that follow the same rule when inflected (generally due to cases). Classical Latin is often said to have five declensions; modern Greek and Russian, three; Finnish, possibly none, except for rules regarding harmony. It is quite clear, then, that the number of declensions does not depend on the number of cases.
I have a problem with Polish.
First, Polish has an unusual amount of genders: masculine, feminine, neuter, masculine/personal plural, non-masculine/personal plural. Regarding declensions related to plural formation, Polish is straightforward. With respect to the genitive singular of masculine nouns, not so much: it really depends on whether of not the noun describes something that feels alive, somehow:
samochód -> samochodu(dead),
widok -> widoku(dead),
chleb -> chleba(alive), but
komputer -> komputera(alive). In the end, this feels alive rule only refers to a certain (not large) fraction of masculine names, since bread and computer are alive, for Poles. Similar controversy is also present in the dative singular, locative singular and, sometimes, genitive plural of feminine nouns.
Second, Polish has a huge repertoire of morphophonetic variations:
widok -> w widoku and
chleb -> w chlebie, but
samochód -> w samochodzie. This happens more often than not:
Praga -> w Pradze,
Marzec -> w Marcu,
gazeta -> w gazecie. Being able to predict such changes is an integral part of learning Polish.
My question regards the following: most mainstream languages I've stumbled upon had some grammarian that divided nouns according to declension, but I've never seen that done to Polish. I have a friend that is a philologist and he said that possibly it is due to the high degree of irregularity present in the Polish language, and that maybe grouping nouns into declensions might generate so many declensions that the problem of categorisation would only get worse. I think that defining declension apart from morphophonetics is possible and feasible, but might be artificial and incomplete. Does anybody know of any reference that groups Polish nouns into declensions?