what is the value of probability P(X)?

X is random variable.

in case of a dice, 

X = {1,2,3,4,5,6}, P(X=1)=P(X=2)=・・・=P(X=6)=1/6

therefore 

P(X) = 1 (trivial result as an axiom of the probability)

the universal set is often written as Ω and P(Ω)=1