I think how much math you need to get into a master's program depends on how exclusive the school is. Most programs really prefer you to have had real analysis and/or abstract algebra, but some schools are willing to let you make those up if you're missing them. I am a math minor and the highest class I will have taken at graduation is discrete math/formal proof, but I have been accepted into mathematics MS programs for next year. The thing that gets a little tricky is funding.
