We should probably speak about three different words that share a common etymology;
«πόσο» means “how much” and it is used when someone asks eg “How much does this cost?” = «Πόσο κοστίζει αυτό;».
«ποσόν» or «ποσό» (we can omit the -ν) means “amount”, eg “A specific amount of money” = «Ένα συγκεκριμένο ποσόν» = «Ένα συγκεκριμένο ποσό».
The third case is «πόσων», genitive and plural of «πόσο», used for example in “How old are you?” = «Πόσων ετών είσαι;».
---------------------------------------------------------------------------------------------------------------------------
Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.