How many graduate-level courses should one take to maximize chances of getting into graduate school? (PhD) This is somewhat similar in broad topic to an earlier question I've asked, but not the same. I intend on applying to interdisciplinary mathematical/computational science/engineering programs like this: https://icme.stanford.edu/ . As a result of the interdisciplinary nature, there are a lot of junior-and-senior-level courses that are relevant, and I want to take as many of these as possible so I have a good grounding for whatever more specific path I follow, within the field of computational science. However, I also know that graduate classes are looked upon favorably by many admission committees; so, should I drop some of the undergrad courses (that may be more relevant in subject matter) and take some grad ones? Because of scheduling and prerequisite issues, I would otherwise not take any graduate-level courses (or take at most one) until my senior year, by when it may not even matter in terms of admissions because some graduate programs don't look at your senior grades)

[As a follow up to this, would it be considered a bad thing to take a undergraduate course that may be considered important or even "crucial" in the final semester of senior year - when admissions decisions are already coming out - because of taking graduate classes earlier on instead?]
Graduate-level courses are helpful, but there isn't any magic formula that "X courses with a grade of A or A- will guarantee admission." Every case is different, and good grades are by no means sufficient for admission, if your letters of recommendation are weak. 

If you're applying for a terminal master's program that is primarily coursework, then the rules are different, but for anything relying on research, your research experience and letters of recommendation will carry significant more weight than your classwork and test scores. What doing well in the graduate courses does is signify that you will be able to handle the coursework in your program, but it does not shed light on the rest of your abilities.

So my recommendation is to take graduate classes because you're interested in the subject and want to learn more about it, rather than just to impress an admissions committee.