So you know that all the manual pages as grouped by sections based on what they do. So what do you do when you want to list all commands at a given section? you tune the apropos command as follows:

apropos --wildcard "*" --section <section_number>

One general knowledge question you can answer is this:

Find how many system calls are supported in linux, at the moment.

The manuals for all system calls are grouped in section 2. The answer becomes quite simple now.

$ apropos --wildcard "*" --section 2 | wc -l
471