The point of a split keyboard is that you don't need to.move your arms, which are relaxed, or your hands. You only move your fingers, and keep your eyes away from the keyboard.

For that, you want such amount of keys which you can comfortably hit by moving just your fingers. It's normally 3 rows and 6-7 columns. And say 5+5 keys in the thumb clusters.

I personally find a layout like Kyria [1] optimal for such use.

[1]: https://splitkb.com/collections/cases-and-plates/products/ky...