Let's take it one point at a time.
Capacitance is defined as the ratio of the charge(Q) of one of the two
plates and the potential difference(V) across the two plates (after
they have been charged, of course)
Capacitance is the amount of charge that can be stored on the plates per volt across the plates, or
$$C=\frac{Q}{V}$$
The statement "after they have been charged, of course" is not essential to the definition of capacitance. This is because capacitance is also defined as
$$C=\frac{εA}{d}$$
where $ε$ is the permittivity of the space between the plates, $A$ is the area of each plate and $d$ is the separation between the plates. Note that voltage is not part of this definition.
and we know that P.D (V) means that the amount of work needed to bring
one electron from one point to another.
Actually, PD is defined as the work required, per unit charge, to move the charge between two points. For a capacitor with constant field $E$ between the plates, the force exerted on a charge between the plates is $QE$ and the work done to move the charge between two points separated by a distance $d$ is $QEd$. Therefore the PD is $Ed$.
So when we divide the number of charges by the P.D, what we are
determining is that how many charges can possibly go from one plate to
another by consuming that PD.
No. Dividing the number of charges by the PD, or $\frac{Q}{Ed}$ makes no sense. Now if you multiplied some total amount of charge times the PD that would give you the total work required to move all those charges a distance $d$. You don't consume PD. You consume energy moving the charges. For a capacitor connected to a battery, the battery supplies that energy.
After they have switched the sides the PD will again get to zero. for
example (it is just an supposition, don't take it seriously) V = 2 v
and Q = 6 then C = 3 c/v which means that the plates can accumulate 3
coulomb of charges. As we increase the V, Q increases too since V =
QR/t (R = resistance) and the ratio never differs unless we change the
physique or vicinity of the plates.
It's difficult to address all of this since it is based on an erroneous understanding of capacitance and PD, which I have already addressed. Please look over my answer and then let me know if your last statement still makes sense to you.
Hope this helps.