Just for the heck of it, I reduced my font on my 5" screen to the smallest it would go. Other than the fact that I can't read it without having my nose pressed on the screen, I get 54 lines. But, it IS readable. Maybe if I had a microscope I'd keep it there ...
When I put it back to where I like the font size, I have 18 lines on the screen. Just means I have to turn more pages.
I agree with Zetmolm, in that probably the reason you don't hear about how many lines you can get on a device is that it is purely arbitrary. I have terrible eyesight (and also have to wear bifocals). So, I make my font bigger. Someone else may have great eyesight and can make their font smaller (and so it'd be more like a paper book ... or smaller).
The best part of any ereader is the ability to make the fonts so it's more comfortable for the reader. Again, IMHO, that's why you don't hear about how many lines a particular reader can/will display.
Hope that helps.
Marilyn