How many milligrams in 10000 mcgs?

This question cannot be answered sensibly. A cubic centimetre (cc) is a measure of volume, with dimensions [L3]. A microgram is a measure of mass, with dimensions [M]. Basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information. It takes only a little mental effort to compare a cc foot of air and a cc of lead and come to the conclusion that the two will have very different masses.