All of the R56 manual transmissions require about 2 Liters of fluid. I suggest having 3 liters on hand, in case you spill some. May I suggest reviewing our transmission fluid DIY article here -
Pelican Technical Article - Mini R56 - Manual Trans Fluid Replacing
It has a lot of great useful information and a step by step process on how to change it out yourself.
R56 Cooper Base transmission fluid here -
http://www.pelicanparts.com/catalog/SuperCat/6346/MIN_6346_TRANSM_pg1.htm#item0
R56 Cooper S transmission fluid here -
http://www.pelicanparts.com/catalog/SuperCat/6344/MIN_6344_TRANSM_pg1.htm#item0