Question

Which multithreading approach determines the context switching?

Answer 1

Context switching itself imposes an overhead on the system, because the OS has to perform book-keeping on which threads exist, which are running, which are ready, which have already received their fair share, etc. In a well-designed OS this overhead is not great, and usually it is not dependent on the number of threads (up to some hard maximum), but it is definitely there.

Everything else is system-dependent. For instance, a scheduler may decrease the nominal length of each slice depending on how many many candidates there are, or it may not. As always, a reasonable answer to decide what to do in your case can only come from running careful performance tests on your system