The sort buffers decides how many pages of data you can sort in each run. That is the basis for the logrithmic function on calculating the number of runs needed to finish the sort.
For example, if you have 500 buffers, then the number of runs is calculated with "log (number of pages in table) with 500 as the log base".
Also note that the number of sort buffers is shared by threads in the parallel sort, if you do not have enough sort buffers, the parallel sort may not work as fast as it should.
