In general, students should only have just 1 CCA. It’s pretty rare to have 2 CCAs because of the time commitment and the workload.
CCA can reduce up to 2 points off your L1R5/L1R4(?) at O levels, depending on what grade you get (A1/2 – 2points, B3/4 – 1 point). It’s graded on the LEAPS system. See this for details.
Most people take the same CCA for the 4 years throughout, reason being you lose all your CCA points if you transfer (or so my school claims), probably unless you have very special reasons, eg medical. It’s not advisable to change CCA and it’s a great hassle (school admins are normally unwilling) so my advice is choose smart.
For 6 year IP, those going on to the affliliated JCs can choose to change, as for schools like DHS/RVHS/ACS/NUSHS, I’m not too sure, probably yes because I heard you get more options at year 5.
Sports/ Perf. Arts requires A LOT of time (training can be 3 – 5 times per week leading up to competitions/SYF/concert) and this will take a toll on your studies and free time. UG is less time-consuming but it’s very disciplined and physically demanding. Clubs is probably the best; not so time-consuming, more relaxing.
If I were you, I’ll try to go for the smaller CCAs to increase your chances of being in the executive comm as this will help to boost your leadership points under LEAPS as well as your portfolio. But of course, interest is the most important imo. There’s no point in joining a CCA and hating it for the next 4/6 years (like me). Best if you could ask the seniors of the school or anyone who is in the CCA you are interested in because it’ll give you a good idea of what to expect.