You could study as many as you wanted to depending on what degree you were taking. For example, if you did science you'd be limited to one language (i.e. I THINK that they are given one arts elective, but you're better off checking the under-graduate handbook at
www.anu.edu.au for specific info relating to certain degrees).
I know in Asian studies, IF YOU ARE DOING STRAIGHT ASIAN STUDIES you can take up more than one language (However if you are combining you have to do a language, and a non-language course unless you're doing a double weighted language). Arts as a degree isnt focussed on languages, so I assume that you can take up more than one even if you are combining it, though I'm not sure.
You get 24 credit points a semester to play with, and on average most subjects are 6 credit points (though Chinese is 12, and Japanese is divided into written and spoken which are both 6 each).
Though, to be honest you are really making it hard on yourself if you do more than one language (especially if you are thinking of combining an arts degree with say, law.)
I'm not sure about early entry, check the undergraduate handbook.