It is mentioned in Fetter's Quantum Theory of Many-Particle Systems (in contraction part of section 8 Wick's Theorem), that:
contractions are c numbers in the occupation-number Hilbert space, not operators.
(c-numbers are just complex numbers, right?)
I am confused because Fetter defines contractions as (time ordered operator) - (normal ordered operator). How does subtraction of two operator become a number? Is it implied that we surround it with $<\Psi_0|...|\Psi_0>$?
Update: See comments for a short answer.
Update 2: See the links in the answer by Qmechanic for a further intuition on defining Wick contractions.