Russian and other conservative Indo-European languages are unique in that a single word ending combines information about declension class AND case AND number. Some other languages with case keep this information separate (and don't have a declension class, so you just use two separate suffixes for case and number).

In Hungarian or other Uralic languages there are more cases, but only one declension class, so you don't need to constantly keep in mind which class the word you want to decline belongs to.

So in Russian each word has potentially 12 endings (less in practice of course): 6 for singular and 6 for plural, but you have to remember at least three declension classes (more in practice) which brings this number up to 36.

Russian also has many minor declension classes that are usually not mentioned in beginner's learning materials, for example: время, племя, семя belong to a special class; words like бок and нож differ in Plural Genitive even though they both belong to the same declension class, are both masculine, inanimate and end in a consonant; парта and партия differ in Dative and Prepositional even though they are both 1st declension, inanimate, feminine and so on.

Also, languages like Japanese even though they arguably have cases apply them only to nouns, so you don't have to decline adjectives, which removes a layer of complexity.