Two or three Subject Tests are required at many colleges. However no college requires the AP. An AP 4 or AP 5 in a subject can lift a student whose SAT Subject Test score in that subject is low. Thus two strategies for ambitious students:
1) Take the SAT Subject Test in every subject that coincides with an AP you are taking. One serves as “backup” to the other.
2) Don’t play to a college’s requirements; instead show your excellence. If you can on a good day get 750+ on an SAT Subject Test, take that subject (and the AP in that subject) even if it’s a third SAT Subject test and your target colleges only require two.
And a third strategy applies if you are reading this with enough time to act:
3) Take AP and SAT Subject Tests as a Freshman and sophomore in any class you are doing well in. You need not wait until Junior year.