Colleges that require SAT IIs require two. Two that recommend but do not require SAT IIs, Georgetown and Johns Hopkins, recommend three; no others recommend three or more. As to what you might take:
Math 2 plus Chemistry or Physics will satisfy the requirements of every college that requires subject tests for every possible major. Any other combination risks a potential preclusion somewhere for at least some majors. Here is what to be aware of:
For colleges that require subject tests, you can usually submit any two for most majors, although it is suggested by some that you submit one that is similar to your requested major.
The exceptions are: (a) even if you are not applying for engineering, Caltech requires math 2 plus physics, chemistry or biology of all applicants and MIT requires any math plus one of those three sciences of all applicants; (b) a number of other colleges require a math, and sometimes the math 2, plus a science for the engineering or science programs; (c) Carnegie Mellon requires a math plus chemistry or physics (biology does not meet requirement) for its engineering college and architecture and requires a math for most other majors; (d) some colleges recommend a math for business majors Also, the UCs which recommend but do not require subject tests, do not accept math 1 and Stanford recommends math 2 if you submit a math.
In other words, taking math 2 and chemistry or physics assures you can apply to every college that requires SAT IIs for every possible major. Nevertheless, if you have no intent to apply for engineering or a science, and no intent to apply to Caltech or MIT, or to Carnegie Mellon for engineering or science or architecture, having math 2 and virtually anything else would be fine. Moreover, if you want to cover all bases, there are some colleges that require a language subject test for placement purposes after you are admitted but not for getting admitted and thus a language could be another test to assure covering every potential.