Carra, Agger, Wilson, Kelly(his best position is CB), Wisdom + new CB should be enough - 6 CB's. We don't need to add there.
I haven't even counted Ayala - 7 CB's
We just need improvement on the current starters and Skrtel will give us the biggest resale value. I would sell and buy better.
We also have Mavinga, McGivern, Sokolik, Regan, Sama and Mendy all as reserve players though further away from the first team.
The best solution I think is to sell Skrtel, Agger, Kyrgiakos, Ayala, Mavinga and Mendy and bring in Cahill & Vertonghen. Should be enough coverage in a season without European football. We can add another if none of the younger have established themselves by the following summer.
It leaves us with
Vertonghen, Cahill, Carra, Wilson, Wisdom, McGivern, Sokolik, Sama
Considering we play around 10 less games next season (none in Europe) that should be more than enough.