"More friends than I" is technically the correct way of saying it rather than "More friends than me" However, "more friends than me" is very common to say and people will understand either usage
...more friends than I. is incorrect in English?
It sounds formal, but is correct. It's a truncation of the form "more friends than I do" which also sounds natural and not so formal. People will understand you with no problem if you say it this way.
In English, there is a big difference between using "I" and "me" here: more friends than I have v. more friends than just me (as your friend). Is that same distinction valid in Portuguese?
Right--excellent point. One is a comparison of how many friends each of us has, whereas "than me" compares how many friends "você" will have in the future versus how many the other person has now.
Para quem tem dificuldade em utilizar o Than/Then.
Recomendo este vídeo: https://www.youtube.com/watch?v=-6PYKm_UCNo
Para quem gosta de conversar sozinho, geralmente utilizaremos o "Then" para substituir o "So/então" "E ai", "E depois".
E o Than é utilizado apenas para comparar.