What's the ideal subdivision count?

There's no one-size-fits-all regiment, but for the average-sized country, I think anywhere between eight and twenty is fine.

The amount of subdivisions a country has should correlate to their history and geopolitics.

I think it depends on the country. We have 31 subdivisions in mainland China, the US has 51 (50 states + DC), but those are countries with fairly large populations and sparsely populated ones may only have a few