This is the 10113th most frequent Spanish word.
centenares
"Centenares" means "hundreds" in English.
The word 'centenares' here is used to estimate a large but undetermined number of birds, emphasizing the quantity metaphorically.
Hay centenares de pájaros sobrevolando el parque.
There are hundreds of birds flying over the park.
'Centenares' is employed to convey the significant number of letters received, indicating its figurative use to suggest multitude.
Recibimos centenares de cartas tras el lanzamiento del producto.
We received hundreds of letters after the product launch.
In this sentence, 'centenares' serves to inform about the extensive collection of old books, being a hyperbolic usage.