So I am going to dissent here. Don't get a second bird UNLESS you actually want two cockatiels. There is no guarantee that the two will get along, and with two birds you are looking at double the expenses, including potentially two separate cages. A number of us keep single 'tiels without an issue.
That being said, if you can't devote at least several hours a day to spend with your 'tiel, I wouldn't get one or two or a flock. You also will need to be prepared to more or less maintain a schedule. A large enough cage and foraging opportunities will also be important.
Here's my other question, how old are you and what is your living situation? A 'tiel is a long-term commitment. They can and do live several decades, and if you are in school, you need to have a clear idea of what is going to happen once you graduate. It's not fair to anybody if you have a 'tiel now, have it bond to you, and then leave it (even with family), because life changes.