This is what I find really crazy about Polish, that so many sentences can either suggest a singular or a plural form. As with the mouse here.
Most feminine nouns have singular genitive=plural nominative=plural accusative, many neuter nouns have it too.
What is unusual about mysz (and some other consonant ending feminine nouns like noc) is singular genitive= plural genitive=plural nominative=plural accusative
Are there that many? I don't try to say that you're wrong, I'm just curious what others you have encountered, because I never thought of that :)