Depends on whether you intend to breed them,a couple of pairs would live happily in there (Gouldians). But if you provide nestboxes you may find one pair is dominant and make life hell for the others....Not fair on the birds, and their happiness should be the priority.
Like humans they may get on...or not.Better to have less birds but happy rather than more birds under stress.
If you're still building the cage go to the trouble of making a central divider so you can clean out one empty side of the cage at a time.Without one you will be trying to clean it with the birds panicking...not good.
Your cage will be twice the size of the lower 4 foot breeder cage in this picture...as you see they all have dividers in the middle to enable cage cleaning etc.
And I'm sorry to disagree but after 40 years of breeding Gouldians I've never seen the addition of another pair limit the aggression between birds.If you want to keep more birds in a small space choose Bengalese (Society) Finches they are the most amiable.
Paul.