For the ones with numbers i think the number is also the reasonable number of players and for the ones without a number i expect it to be 2. I tried some galaxies and checked it with the amoebia and some of them are ... aehm, "strange"? 3 players directly next to each other e.g. (twin galaxy with 6 players)



Which number of players is suggested for which map+size combination?

There isn't really a suggested number of players for any map, but I think some maps have a limit to the max number of players you can have (lower than 8). Also, the placement of an empire in the map depends on it's color, not the number of players.