Could someone explain the inflection of πόσος and its meaning.
In the singular, it means "how much" (e.g. πόση μπίρα "how much beer"); in the plural, it means "how many" (e.g. πόσες αδελφές "how many sisters").
For the inflection, have a look at Wiktionary: https://en.wiktionary.org/wiki/%CF%80%CF%8C%CF%83%CE%BF%CF%82#Declension