Careers adviser at my school said that anyone doing further maths should take other subjects (including maths) just to keep their options open a little bit more but this is obviously not compulsory. I do maths, further maths, physics and chemistry (self studying further maths) and I would suggest doing another a level. Chemistry is the one that fits in the best but maybe a contrasting one like history? Again, not required but useful.
If you have any courses in mind, it might be useful to look at the entry requirements at different universities for that particular course and try find out what the most common taken a levels were for entrants on the course. I think Whatuni has this information but it may be the UCAS website (if neither of them, someone should correct me and say which one it is)