I love the responses from you guys! Shows you're a developer who cares.
I feel like a row of 15 straight across would be great, and limit Fall Mountain to only be chosen if the remaining players were 15 or less. Hex-A-Gone can support up to 20, and people really enjoy that mode, so limiting Fall Mountain to 15 would allow Hex-A-Gone to appear more if the final number were greater than 15.