Someone "reported" this post as Spam -- do get my attention?
Anyway -- the relevant setting (for how many recent folders to show) is in app settings under Account list (but also affects the navigation drawer).
The FAQ has more stuff about folders too, I recommend reading it.
And no, the app doesn't show *all* folders at once -- and this was a deliberate decision on my part (I hate seeing all the weird "[Gmail]" folders when I use Thunderbird, for example).