Learn Polish: Pięciu wyborców właśnie głosuje. - Five voters are voting right now.

This is the core rule for numbers 5 and above in Polish. When a number is 5 or more, the entire phrase acts like a singular block or quantity. Because of this, the verb must take the 3rd-person singular form (głosuje), not the plural (głosują).

Why do we use pięciu instead of pięć here?

Wyborca (voter) is a masculine personal noun. When a number from 5 upwards is the subject for a group of men or a mixed group, you must use its special masculine personal form, which is pięciu. If the noun were an animal or object, you would just use pięć.

What case is wyborców and why?

It is in the genitive plural case. Whenever you use a number from 5 upwards, the noun that follows it must be in the genitive plural. You can think of the grammar as expressing a quantity of something, similar to saying a group of five of the voters.

The noun wyborca ends in -a, which usually makes a noun feminine. Why does it take the masculine ending -ów here?