It is intended that you cannot put more than one Dreadnought into a single army.

This is because Dreadnoughts gain veterancy bonuses that would otherwise be too powerful when combined. For example, you could put in a Dreadnought that gave +20% damage to your army and then put another Dreadnought into the same army. This Dreadnought would also gain the +20% damage bonus from the first Dreadnought on top of its own Veterancy bonuses (Such as increased attack speed or armor penetration).

The result would be that if you added 4 or 5 Dreadnoughts in the same army, you'd end up with a strategically unbreakable army (Imagine all 5 of them having regeneration aura's).

I'd also like to add that - while having multiple Dreadnoughts in the same army would be overpowered - having only Dreadnoughts in a single army (No cruisers or anything like that) makes for a very vulnerable army. Dreadnoughts by themselves are very bad against air (But really great when paired up with anti-air cruisers due to the bonuses they provide).

This is because bombers always ignore armor and their attacks are in burst. Dreadnoughts are also big targets so a lot of bombs from a single bomber would hurt it a lot.


Edit: The vulnerability aspect is also true for enemy artillery. Because radar coverage would keep the small artillery units out of your slow Dreadnought's attack range. The artillery would normally fire at whatever is closest to them. Dreadnoughts in an army of a lot of small units would mean the small army would be the targets (Because Dreadnoughts are typically in the middle)

So with only Dreadnoughts, the artillery would always hit the Dreadnoughts. In competitive games, Dreadnoughts are the ideal artillery targets.