Physical buttons 1-6 in a horizontal row in car stereos are placed there because:
- It's easy to save a station without looking at the stereo
- Physical buttons give tactile feedback and can be found easily without looking at the stereo

It doesn't make sense to arbitrarily limit the number of stations you can save just in order to emulate a car stereo interface using a touch screen. The current UI is really easy to figure out, there is no need to change it again.

One thing that would be nice to have is when you tap and hold the bottom button while the frequency scale selector is at an unsaved station position, then it should automatically save that position as a station (exactly like a single physical button on a car stereo would do).