J C Lawrence
I make it a point to track the recommended # of players for all my Age of Steam boards.
There are no official values. The following geeklist is about as close as it gets:
I have the London map listed as 3-4 players, but I have no listing for the Sun map.
London was designed as a 4 player map. It works with 3 players, but I think it is best with 4. Some players like it with 5. I like London best with 4 players.
Sun was specifically designed as a 3 player map. Some like it better with 4 (which was also a design target after 3), and some also like 5. I like it best with 3 players.
Is there an "official" recommended # of players for this map?
Writing as the designer: No. Player tastes vary. The things I optimise for at specific player counts reveal more about my preferences in Age of Steam than anything else. Your preferences may be different. Select your own counsel, make up your own mind, it is your decision.