SCT
 

Ok this might be a crazy question , But when you get blood you most of the time get 1 unit or 2 ..... when you need platelets you get a unit ... How do they know how much stem cells one needs when everyone size age is different ??

They do a calculation based on body mass. It is a crude calculation, and I think that most of the time they err a little toward the higher side if the donor's marrow/stem cell collection affords it.

I have seen people receive 3 units of blood and 2 or 3 units of platelets depending on the circumstances.

The stem cell collection is an actual calculation, the PRBC and Platelets are probably an experience based thing, depending on the patient and the center.