There were enough of them already, then Oda introduced a bunch of new ones. Some upcoming marines like Drake, Koby or Grus might get promoted to VC soon.
Why are there so many of them, with distinct designs and fighting styles? When and how will they be utilzed?
Obviously there will be a New Marineford war afte Elbaf. In the original MF war, the allied captains of WB fought the VCs. Luffy has fewer captains in his Grandfleet, and will be opposed by alot more VCs. So maybe every Strawhat gets his own.
For example: Nami vs Blue Grass (she looks a little like Nami's BB opponent Bakkin)

Why are there so many of them, with distinct designs and fighting styles? When and how will they be utilzed?
Obviously there will be a New Marineford war afte Elbaf. In the original MF war, the allied captains of WB fought the VCs. Luffy has fewer captains in his Grandfleet, and will be opposed by alot more VCs. So maybe every Strawhat gets his own.
For example: Nami vs Blue Grass (she looks a little like Nami's BB opponent Bakkin)