"Can I count with you" was incorrect. Wouldn't it be Eu posso contar no voce? to be, Can I count on you?
"Can I count with you?" is translated the same way as "Can I count on you?" in Portuguese. In English they have different meanings and in Portuguese you notice that by the context. One thing to keep in mind is that sometimes "verb+preposition" does not match in English vs. Portuguese and you just have to know them by heart. "Do you agree WITH me?" = "você concorda COMigo?" ( same preposition). "Dont worry ABOUT that" = "Não se preocupe COM isso" (Now it is different). It is the same for "to count on / with" that in Portuguese are both translated as "contar COM". ;)