This is the 2163rd most frequent Turkish word.
kaç
"Kaç" means "how many" or "how much" in English.
In this context, 'kaçan' is derived from 'kaçmak' (to escape) and means 'who escaped' as part of a participle construction.
Kaçan adamı gördün mü?
Did you see the man who escaped?
In this sentence 'kaç' is used as an interrogative word meaning 'how many/much' to ask about age.
Kaç yaşındasın?
How old are you?
Here 'kaç' queries a numerical quantity, related to the noun 'books'.