Keep in mind in the cycling world (as well as real life), there is the N+1 phenomena.
So, whatever you have N of, inevitably you will need N+1.
So, say you have 20 speeds (10 in rear x 2 in front), there is always that temptation to go to 22 speeds (11x2). Or, 12x2 (or 12x3???)
In your case, if everything is mostly flat, then you may not actually require much.
But, in general there are two main driving forces for more sprockets.
- Tight gearing and small steps is nice. Keep it in the best gear of the moment.
- Range of gearing. Best to keep power from fast shallow descents up to steep climbs.
In the past racers were forced to choose between either tight gearing or wide range gearing. They no longer are forced compromise as much between the two somewhat independent needs.