How would you differentiate here between "I wrote many long letters" and "I wrote a very long letter" besides "list" being plural in the former?
"Napisałem wiele/dużo długich listów".
In the original sentence you have Accusative (even if it looks like Nominative), but wiele/dużo would require Genitive.
Why is it napisałem when its a female voice ? ,
Disregard the male/female voice, they're random. We have no power to make a masculine sentence read by a man and a feminine one read by a woman.
You may treat it as just "someone reads a sentence that was written somewhere".
Załęże od konteksta- moźemy również używać tutaj czasu Prezent Perfect - I have written him...
"Zależy od kontekstu" :)
Możemy użyć Present Perfect, ale tutaj nie ma żadnego "him" - można za to napisać "I have written a very long letter".