Two, my native and English, though I do understand Serbian/Croatian/Bosnian perfectly as well. I do speak them, but not as good as English.
My grandpareents... IDK, probably 3 I guess... tops. Slovenian, Serbian, Macedonian. My granma from my mother's side also knew a little Turkish and was quite fluent in German (she was in a labour camp during WWII). My kid is little, like 4, he speaks Macedonian and some English (words only, not sentences).
Well, yeah, our local ones, we do usually, at least, understand what our neighbours are saying (Albanian is an exception, it's not a slav language), and English nowadays. Used to be French or Russian back in the day, in the 60s, but at the end of the 60s, Yugoslavia had a good thing going with the US and a lot of things got centered towards the US, including culture (there was a hippy movement in Yugoslavia as well, not to mention YU rock, that's a subgenre on it's own) and which foreign languages were taught in school. English became the default at the end of the 60s, begining of the 70s.