**You can take as many electives as you want (more than the degree required electives i.e. 5 for BS, 6 for BA)
The COMP electives are the courses numbered 420 and higher (COMP 496, COMP 690, and COMP 692H don't count as credit towards the CS undergrad major). Many ask what electives they should take. This depends on your tastes, there are a wide variety of courses offered. I still advise you to take a bit of everything, or if you are sure that you want to specialize you can pick courses that are of a sub-field (Software Engineering, AI/ML, etc).
Try to remember what you have enjoyed from the core courses. Then if you want, you can take similar courses. For example, if you enjoyed COMP301, you can take COMP423, COMP426, COMP433, and COMP523. If you enjoyed COMP 211, you take COMP 530/630. If you enjoyed COMP311, you can take COMP541. If you enjoyed COMP455, you can take COMP520, COMP524. The example goes on and on.
How difficult are the electives? Many are difficult, many are easy, and this depends on the professor and the nature of the course. These are courses in the 400, 500, 600, and 700 levels, so they can be a bit hard.
Follow this on choosing courses.
Check the Study Strategy.