There were four main sieges during the war, and a host of lesser known sieges. The four main sieges were:
Kimberley - 14th October, 1899 to 15th February, 1900
Ladysmith - 3rd November, 1899 to 28th February, 1900
Mafeking - 13th October, 1899 to 17th May, 1900
Wepener - 9th April, 1900 to 25th April, 1900
Clasps were issued for the defence and relief of each of these sieges, (apart from Wepener which has no relief clasp) which are popular amongst collectors. Another angle is that of collecting defence and relief combinations and double relief combinations. From the data collected for The Register these are the available combinations and known medals to date are:
Defence of Kimberley - Relief of Mafeking : 153 (420 - Kaplan)
Defence of Ladysmith - Relief of Mafeking : 18 + 18 (33 + 184 with Elandslaagte clasp - Kaplan)
Relief of Ladysmith - Relief of Mafeking : 127 (209 - Kaplan)
Relief of Kimberley - Relief of Mafeking : 7
Relief of Kimberley - Relief of Ladysmith : 2
Relief of Kimberley - (Defence of) Wepener : 3
There are no "double defence" combinations.
"Kaplan" refers to "The Medal Roll of the Queen's South Africa Medal with Bar Relief of Mafeking, 1980".
As more clasp data is input into The Register these figures are likely to increase, but I believe the relative scarcity of each combination will remain.