IMO I feel it's best to always have cockatiels in pairs. Maybe not at the same time at first though. Unless you can devote enough time to just one each day, I feel that only one would get quite lonely. But like others said, it is really something you should investigate before jumping into.
edit: forgot, it's a good idea to quarantine for a month or two if you get more than one at once. Although, that's usually if you're adding a bird when you already have one in your house. Some people say it doesn't do much as you can't seal off the air, but it helps if the new one is sick as birds hide their illness and it can be very hard to detect at times if one is not to full health.
They can cost a bit to keep up. You should take them to the vet yearly for a checkup. Oh, you should look up and get an emergency vet that sees exotics. You don't want to be scrambling to find one after an accident has occurred or if your bird is suddenly ill.
They also require a good size cage. If you have two, I would say keep the cage at least 2.5 feet wide, pref larger if you can.
You also need to be very careful of fumes for all birds, but esp the smallers ones. No non stick pans, febreeze, any sort of furniture polish.. anything that's scented - keep that away from them it can be fatal. Watch out for poorly made toys that could get nails tangled or might have zinc in the metals as zinc can poison birds.
They love veggies. Mine love carrots in particular. But there are certain things you should never feed. Onions, anything leek, fruit pits, strawberries can contain a toxic mold so be wary of those... avacado is deadly. Never ever feed them that. Here is a great list of things you can feed your bird:
http://www.cockatielcottage.net/tablefoods.html
I love cockatiels. They are my favorite and most cherished bird. They can live long lives and make wonderful friends. You need to give them lots of attention, toys, a diet of not just seeds (not like 70% of pellets either - I didn't know this until recently..). They all have unique personalities and their own little quirks and silly habits. Both males and females make lovely pets. Males tend to be more vocal though.
I wish you best of luck on your bird journey.
