I know English, Mandarin (most common Chinese language), and am picking up Japanese. I can converse in all 3, so I voted 3, but due to lack of practice, two of them are rather difficult for me.
I speak rudimentary Hokkien to my grandmother, and used to speak rudimentary Indoesian Malay to a domestic helper (but we no longer have one).
I must say... If you know Mandarin and English, the next language to pick up should be Japanese. There are a large number of these two elements in Japanese, so that will assist you tremendously.