As long as you are taking the prerequisites for the program you want and have the average, you'll be fine. It doesn't really matter what faculty you end up in. The actual program is what matters when you go through your university career. So to answer your question, you can select the math program you want just as you selected a humanities one. Remember when you are picking your majors, you can apply up to 4 majors so select carefully and make sure you have all the requirements.