Approximate quantities (en)

To estimate a quantity in Swedish, place the singular article en directly before a number. While en normally means "a" or "one", when paired with a plural amount, it translates to "about" or "approximately".

About fifty demonstrators shout the slogan.
En femtio demonstranter ropar slagordet.
At the demonstration, about fifty demonstrators are standing and shouting the slogan.
Vid demonstrationen står en femtio demonstranter och ropar slagordet.

Always "en"

When used this way, the word is always en, regardless of whether the plural noun is an en-word or an ett-word. The article acts as an approximation marker for the number itself, so it does not change to match the noun's gender.

About thirty riot police officers are standing and waiting with their shields in front of them.
En trettio kravallpoliser står och väntar med sina sköldar framför sig.
About twenty peaceful demonstrators are carrying placards and banners.
En tjugo fredliga demonstranter bär plakat och banderoller.

These approximation phrases act exactly like normal noun phrases, meaning they can function as both subjects and objects in a sentence:

The demonstration became violent when about forty people started fighting.
Demonstrationen blev våldsam när en fyrtio personer började bråka.
The riot police let about thirty peaceful demonstrators carry their banners.
Kravallpolisen låter en trettio fredliga demonstranter bära sina banderoller.