"Sixty people go to prison."
Translation:Sześćdziesiąt osób idzie do więzienia.
Okay, this is gonna be a bit harder to grasp. Basically, the correct numeral if you were to use "ludzi" is... "sześćdziesięcioro". It is a collective noun, and this is the moment where learning Polish gets pretty hardcore. Collective nouns, according to wiktionary, "are used before nouns denoting people of mixed sex, personal and animate neuter nouns (of which there are few), and non-masculine personal pluralia tanta (words that have only a plural form)".
While 'osoba' may mean a male or female person, it is grammatically feminine. Professor Mirosław Bańko, one of the best recognizable Polish linguists, says: collective nouns, when signalizing mixed sex of the referrents, may be used only with plural masculine.
Therefore we need to say 'sześćdziesięcioro ludzi", but simple "sześćdziesiąt osób" as in this example.