This is the 9926th most frequent Polish word.
apartamentów
The Polish word "apartamentów" means "of apartments" (genitive plural form).
In this sentence, 'apartamentów' is in the genitive plural case because it is part of a phrase indicating quantity ('liczba' - 'number').
Liczba nowo wybudowanych apartamentów w centrum szybko rośnie.
The number of newly built apartments in the center is quickly increasing.
Here, 'apartamentów' is in the genitive plural case, as it is used to show possession ('projekt' - 'design' of the apartments).
Projekt nowoczesnych apartamentów zdobył nagrodę za najlepszy design.
The design of the modern apartments won an award for the best design.
In this case, 'apartamentów' is in the genitive plural case because it is used in a negation ('brakuje' - 'lack').