Translation:One day you will have more friends than me.
"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
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.
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.