The more the better however as was previously stated they should be unique and bring a different feeling. I would prefer more hubs but smaller. I would like the hubs to be on a planet and not in orbital stations. I want to get the feeling of the planet, the culture and the people that live there. I wouldn't miss the Citadel at all.
Hope for a system close to Mass Effect 1 where every world has a small hubs with merchants and small sidequest as well as main ones. You leave the hub when you go on planetary exploration. You visit it, you get the feeling of place, you do missions on the planet and then you move on the second planet where you visit another hub with different feeling and missions. By using this structure you can do mainly two things
1) Have planetary exploration integrated into the main mission (maybe with vehicles)
2) Have more but smaller different hubs on each planet. Each hub should be fresh and give you the feeling from the planet in which you are in (from a developed world to a mere outpost)