Learn Polish: Kiedy przyjechaliśmy, widzieliśmy sześć osób.

This is a major rule in Polish: numbers 5 and higher work differently than 2, 3, and 4. They act like a quantity, requiring the following noun to be in the Genitive plural (translating roughly to 'six of people'). That is why we use osób instead of the basic plural osoby.

How exactly does the word osoba change into osób in the Genitive plural?

For most feminine nouns ending in -a, you form the Genitive plural by simply dropping the -a. So, osoba loses its -a. However, Polish often changes the letter 'o' to 'ó' when it suddenly becomes the last vowel in the word. That gives us osób.

Why is przyjechaliśmy perfective while widzieliśmy is imperfective?

Przyjechaliśmy (we arrived) is a completed, one-time action. Widzieliśmy (we saw) is imperfective because it describes an ongoing state or background situation right after they arrived. They arrived (done), and as a result, they were seeing six people.

Does the ending of przyjechaliśmy and widzieliśmy tell us who the 'we' is?