Why is Wick contraction a $c$-number? 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.
 A: Comments to the question (v3):


*

*The main fields assumption that goes into the proof of the Wick's theorem for fields $\hat{\phi}^i\in{\cal A}$ is that their (super)commutators $$[\hat{\phi}^i,\hat{\phi}^j]~\in~ Z({\cal A}) \tag{1}$$ are central elements of the operator algebra ${\cal A}$, cf. e.g. this Phys.SE post.

*For free fields $\hat{\phi}^i\in{\cal A}$, their (super)commutators $$[\hat{\phi}^i,\hat{\phi}^j]~= (c~{\rm number}) \times \hat{\bf 1} \tag{2}$$
are proportional to the identity operator $\hat{\bf 1}$. Under mild assumptions one may prove that the contractions 
$$\tag{1} \hat{C}^{ij}~=~T(\hat{\phi}^i\hat{\phi}^j)~-~:\hat{\phi}^i\hat{\phi}^j: ~=~c^{ij}~ \hat{\bf 1} $$
are $c$-numbers times the identity operator $\hat{\bf 1}$, cf. e.g. this Phys.SE post. Be aware that 
in the physics literature, the word  contraction sometimes refers to the operator $\hat{C}^{ij}$ and sometimes it refers to the $c$-number $c^{ij}$.

*Eq. (1) does not necessarily hold for interacting fields, and the corresponding contractions and Wick's theorem get modified.
