Because napisała is feminine. “He wrote” (he being any masculine noun) would be napisał and “it wrote” (subject of neuter grammatical gender) would be napisało.
On a side note, the sentence in some really odd contexts would actually translate to “it wrote only one letter” in English (eg. Książka. Napisała tylko jeden list. = “A book. It wrote only one letter.”, because a book is feminine in Polish) but I cannot come up with anything grammatically correct that makes it translate to “he wrote”.
Yes. And it has actually pretty good reason – the past tense formed out of what earlier had been active past participles, which would decline just like adjectives. (And instead of he wrote it one had to say he he-who-wrote is, on napisał jest in Old Polish, or I he-who-wrote am, ja napisał jeśm in Old Polish. Those lost their old to be verb and also their adjectival flavour, but the inflection by gender remained. ;-))
So is it incorrect/uncommon to say "Ona napisała tylko jeden list" given that the information about gender is already there?
Interesting... as far as I remember in Slovak (maybe in Czech too?) this old formula stayed.
Can you also say she only wrote one letter or does it have to be translated as has written?
Well, "a" is not a numeral, they're technically different even if they say more or less the same thing. Actually, "only a letter" and "only one letter" are quite different, I'd say. "only a letter" seems to mean something "it was only a letter, not a book!"
so as in english one letter from a word has the same form as one letter that you send to a friend?
I am not sure what you mean... do you mean the word "letter" (A, B, C etc.) vs the word "letter" (written to a friend)?
The first one is "litera" in Polish.