Personally, I always prefer to have at least 2 of one species. I don't like the idea of depriving a pet of their own species, although people do keep single pets and they do just fine. If your future cockatiel is going to be alone most of the day, I would recommend getting two. Cockatiels are very social birds and get very lonely if left alone for long periods of time. I have two and have never regretted getting a pair. People say pairs will bond to each other and not as much to the humans, but in our case its not true. They are bonded to each other as well as to us, so we have no problem like that.

One thing I would like to add is, get 2 males or 2 females if you do not want to deal with future babies. Although a male/female pair will not always mate and get chicks, its a possibility, so if you want to get 2, either get 2 of the same sex, or a male and female with a cage of their own, beside each other. If you do want baby tiels in the future then it shouldn't be a problem. Good luck!