Let's suppose for argument's sake that the average attack leads to 6 hours of protection. And let's suppose the game has 100 players in each faction. The average player will be able to get in at most four attacks on live bases per day, and the rest will have to be dev bases.
Now what if the game gets wildly popular, and there are a hundred trillion players in each faction instead of just a hundred? Well, it's the same story: you have lots more bases to attack, but there are lots more players attacking them. The math still works out the same -- if the average attack leads to 6 hours of protection, then you have about four attacks on live bases per day per player before it goes to dev bases.
It doesn't matter how many players there are. The game can become the world's biggest blockbuster, but if the amount of protection each attack generates doesn't change, the number of dev bases won't change.
One thing that will affect it is inactive players. With 100 players and attacks generating 6 hours of protection, you get about 400 attacks on live bases per day. If 90 of the players only play in squad wars, then the other 10 get to split the 400 attacks -- they each get to make 40 attacks before they go to dev bases.
Now, what if the factions are imbalanced? Suppose one faction, let's call it Faction E, doubles to 200 players, while Faction R only has 100. Faction E still only gets 400 attacks before they get to dev bases, but now it's split across 200 players -- they get two apiece. Faction R now has twice as many people to hit, so they have 800 attacks to split across 100 players, so they get eight apiece. That makes a real difference if the imbalance gets that large, but small imbalances won't be easily noticed. But it isn't the global balance that matters; it's the balance on any one planet -- if everyone from one faction clusters on a single planet, then things will be easy for them on that planet but very hard on all of the others.
Long story short (too late for that, I suppose):
- The number of players doesn't affect how many dev bases there are; don't lose too much sleep about whether the game is gaining or losing players.
- Player activity does affect it -- if most players don't play PvP much, then the ones that do have lots of live bases to hit.
- Faction imbalance doesn't affect it much at small imbalances, but does affect it a lot at large imbalances.
- A faction clustering on a single planet will make an imbalance much stronger.