By plates I assume you mean for a finite element model?
If so, I would recommend getting thing as square as possible (6x2 has an aspect ratio of 3:1, which isn't great for FE models). I would also recommend that you do a couple models, one with a coarse mesh, one with a finer mesh, and see how much the results vary. That should give you an idea of how they are converging. If there is a big difference in the results, add a new model with an even finer mesh, an so on. At some point, the results won't change much. That's when it's probably OK to stop.
If you mean actual physical plates, you can approximate by dividing the tank area by plate area and add 5%-10%. Or you can lay it all out. Actual plate sizes and layouts are based on economic considerations, including equipment available and plate sizes and costs available.