Different regions have different exam requirements. It all depends on what the local authority/department/administration has decided they want to have. In some cases a higher level licence's exam(s) can be sat instead of a lower licences exams, in other cases that's not allowed. It largely depends on exam content
FAA: One exam per licence. You would typically do a PPL exam, then a CPL exam and finally an ATP exam.
Australia: Has varied over the years. In the early '80s it was PPL=4 (Basic Aeronautical Knowledge + Nav, Met & Law only after BAK was done), CPL=6, ATPL=4. CPLs could replace 3 of the PPL exams but ATPL content was too different & didn't cover a lot of CPL stuff so couldn't be used as a PPL or CPL subsitute.
Later Oz changed PPL to BAK + a single combined exam; CPL to 7 exams (original 6 as multi choice + a written final exam) and later again to a single exam; and ATP to a single combined exam.
Now they've gone back to multiple exams for some licences. Still the same crediting process though: CPL is OK for PPL but ATPL stands alone.
UK: UK used to have a number of exams for PPL (4?), more exams for CPL(was it 6 or 8 or something? Dunno - I did a UK ATPL after my foreign ATPLs) & 14 for ATPL. ATPL could sub for CPL *and* IR exams because a lot of the content was similar, so that's what most people did. To do otherwise meant doing the CPL & IR exams and then later having to do the ATPL exams prior to being able to upgrade to an ATPL.
JAR: Similar to UK