This is the 468th most frequent Polish word.
mnóstwo
A great deal; a lot.
In this sentence, 'mnóstwo' conveys the meaning of 'a large quantity' or 'many' and is connected with the noun 'książek' (books) in the genitive case.
Mam mnóstwo książek do przeczytania.
I have plenty of books to read.
'Mnóstwo' here indicates an abundance of 'ptaków i wiewiórek' (birds and squirrels), again showing its function to denote a large number and requiring the genitive case for the nouns it modifies.
W parku było mnóstwo ptaków i wiewiórek.
There were numerous birds and squirrels in the park.
In this example, 'mnóstwo' represents 'a significant amount' and is used with 'czasu' (time), illustrating its use to quantify something abstract.