Whether you should buy one budgie or two (or more) really depends on whether or not you plan on taming and having a close bond with your budgie. Budgies are flock creatures and their entire world is their flock. A budgie without a flock is an incomplete, lonely budgie.
If you do plan on taming and bonding with your pet budgie, then it is perfectly okay for you to only get one budgie. You can provide your budgie with all the social interaction and support that it needs. In this case, you (and your family/house-mates) will become your budgie’s flock. This is why one budgie kept alone will bond so closely with its owners; the budgie’s people are its flock. As long as you can provide your budgie with companionship, you can keep him/her as a single budgie.
If you don’t plan on bonding with your budgie but would like either a flock of tame budgies or don’t want to interact with your budgie(s), then you do need to buy two or more budgies to be kept together. The budgies kept together will form a flock and can keep each other happy and fulfilled. They can still be tamed if you wish, but it will take some extra work and most likely none of them will form a particularly close bond with you.
Personally, I find it is much easier to bond with your budgie and tame him/her if you only have one. I always recommend only buying one budgie at a time if you want it tame, and absolutely if you want it to talk. Two budgies bought at the same time will almost never learn “people talk”. That being said…
It really isn’t true that is if you have a budgie and you get a second, or you get two budgies that they will ONLY prefer each other and not bond to their owner.
It is true that two budgies will look to each other for companionship, but they not only can bond to each other, but to the owner as well, that is if the owner pays attention to them and works with them and the birds trust him or her.
When you have two budgies, the bond to you may not be as tight as the bond of a single budgie to you might be (a single budgie will view the owner as his mate, thus bonding tightly with him/her). We first had a female budgie and she bonded tightly with my husband. Later on we decided to get her a mate, a budgie male, as she was trying to make a nest in my laundry basket. This female then bonded to her male budgie, even had chicks with him, but she remained also bonded with my husband. She would still fly to his shoulder, and sit on him. The male budgie just followed the female.
(I feel having first only 1 budgie is best, then adding a second budgie after the first is firmly bonded to you.)
So I really do think it is a misconception that two birds only will bond to themselves and not the owner. Budgies, and other parrots have the capability of developing several and various bonds throughout their life time.
What kind of relationship you have with your budgie or budgies, depends really on how much time and attention you give them. One normally gets out of a relationship what one puts into it! I firmly believe that!