HL of IB stands for higher level and SL stands for standard level.
IB Mathematics Research SLI should be translated into Chinese as "College Mathematics 1".
Every IB student needs to choose at least three HL's and three SL's, and finish their studies with CAS, TOK and EE, which is not what the upstairs said. Of course, there are also confident students who choose 4 HL, but personally it is not recommended. Because this will not only greatly increase the number of courses, but most importantly, it will not be of much help to college applications. Remember this, your grades are more important than your courses.