I also want to go into dentistry and I'm in the first year of sixth form taking chem, bio, maths and psychology.
Maths is a good subject to do but I would definitely only do it if you like the subject and think you are confident enough. However, I don't think maths is necessary even though a lot of people applying for medicine do take it. I would make sure you are completely happy when picking another subject and never take something that parents want you to do. I am taking psychology because it is regarded as a science subject and it isn't too difficult. Also physics contains a lot of maths and I know people who take it without taking maths at a level and they say its fine but a large chunk of the exam is maths so you just need to be comfortable doing it.