You need at least probability/statistics, linear algebra, calculus, and numerical methods. Once you know this machine learning is a relatively thin layer built on this math foundation. The real problem is learning the math foundation.

It might be enough to be able to use tools that other people have built, and have a rough idea of what's going on under the hood, and how to select which algorithm to use in a very general sense. It won't be enough for you to be designing your own algorithms.