I've had 2 birds for over 4 years and just added a third in October.
As far as feeding on a daily basis, not much of a difference. I have to prepare the food anyways, so I'm just preparing for one more.
Cleaning takes a little longer. I was already cleaning and changing paper in 2 cages, so its an extra bit of time.
Where I struggle sometimes is out of cage time and attention since I'm the only person in the house that handles the birds and I give individual time out - Even with supervised time out together I don't trust them yet. The routine is getting easier though. Each bird has at least an hour of out of cage time each night and a couple hours a day on weekends. I spend time interacting with them and then they have their own time to hang out on the jungle gym. That's when I catch up on some TV or I'm on the laptop.
As entranced mentioned, once you have one bird then that is usually the biggest adjustment. Adding additional birds adds incremental time to cleaning/feeding.
I will say that I would never get more than 3 as long as I'm working full time - that's my limit. Of course, if we were to get lucky and win a lottery, there would be a bird room in our new house equipped with TV and computer and I don't think I would ever leave the room
