Is it possible to use a remote with more than four buttons as a trigger for the SmartThings Smart Lighting app? Do I need to add a [number of buttons] attribute to the device type somewhere to enable this? Right now lighting automations seem to assume that every button device has exactly four buttons, which isn’t useful when the button numbers coming off of the device are 2-6 
It won’t accept buttons other than 1-4, but it does allow pushed vs held to get 8 effective button options. You could do this in the devicetype by converting 2-6 from the device into 1-3 pushed, and 1-3 held in whatever pattern makes the most sense.
It stems from the the first and most common button remote, the Aeon Minimote, which has 4 buttons that can register a push or a hold. The 8 events have become a sort of standard in SmartApps including Smart Lighting because it keeps it very simple. Most of the time, the creator of the devicetype for a new button device will map the button presses into these events to work with Smart Lighting.