I took 3 humanities classes the whole time. 1 semester of English, Naval History, and Asia and the Modern World. I hated the non-major courses and wanted to take as many major electives as possible. Helped with service assignment too.
I've said this before, but the USNA validation page on the website is not exhaustive; it just covers Plebe courses most mids coming from high school would have questions about. Course validation policy is determined by the department and varies.
For folks coming from college, credits don't transfer, but that doesn't mean you can't validate every single course you just took in college and get that time back for free electives, semester abroad, VGEP, or dual major.
Validation opens up options. You always want options. Don't make the assumption Calc will be easier because you already took AP Calc in high school, it might still suck just as much, and now you have to do three semesters of it instead of two because you didn't validate.
9 credits, 3 classes will make semester abroad or VGEP feasible if you take summer school one summer and/or overload one semester.
12 credits, 4 classes will more definitively clear a semester, with summer school or overloading probably optional depending on major.