I actually find it quieter to have one tiel! LOL, when I only had one, she was only loud when we left the room without her, since we were her flock. But now that I have a flock, they squawk up a storm in the mornings! They aren't wanting anything, they just like being noisy. They settle down after a few hours though, especially if I'm in there and talking to them and rubbing their heads.
It honestly depends on the bird, though. Because even in the flock, my first tiel, Cookie, is mostly silent. She's just not much of a talker. She's also very much a loner, and prefers it when I take her out of the room to spend alone time with me or when I'm just rubbing her head, trying to ignore the others wanting their share. If you have at least three hours in a day to spend time with your tiel, it won't hurt to get just one. They will just want to be your companion for almost everything, always riding around on your shoulder and getting into your business. But if you have school or work, just like every other human, then get at least two. Granted, they will still need attention. Tiels, from my experience, are very "flock oriented" birds.
Like I said, this is just my opinion. I've had tiels for twelve (almost thirteen) years now, so I'd like to think I have at least some experience with them.