Well, as you've already said, any new bird should be quarantined for 45 days in a separate room anyway. So you'll need an extra cage.
After the 45 days, you can introduce them all to each other. If they get along really well, you can consider putting them all in the same (big) cage. I know quite a few people who keep more than 2 budgies together successfully. You just need to be prepared for the possibility that they won't get along, in which case you'd have to house them separately. But if you had a big enough cage, you probably wouldn't have any problems. My understanding with budgies is that if they're fighting, it's *usually* because the cage isn't big enough.