Breakdown of Hay una cincuentena de personas, y la puerta está cerrada.
de
from / of
y
and
una
a, an (feminine)
estar
to be (location/state)
Questions & Answers about Hay una cincuentena de personas, y la puerta está cerrada.
How is una cincuentena different from just saying cincuenta?
Adding the -ena suffix to a number turns it into a collective noun. While cincuenta means exactly fifty, una cincuentena means 'a group of fifty' and is almost always used to mean 'about fifty' or 'some fifty'.
Why is there a de before personas?
Because cincuentena is a noun, not a regular number. Just as you say 'a group of people' in English, you need the preposition de to connect the collective noun to the noun it groups together (una cincuentena de personas).
Can I use this -ena suffix with any multiple of ten?
Why does this sentence use está cerrada instead of es cerrada?
We use estar with a past participle (like cerrada) to describe the current state or condition of something resulting from an action. The door is currently in a closed state, which is not an inherent characteristic (which would require ser).