I would start with one, unless you see a bonded pair in need of a home - if you don't think it's too much to take on. It's my opinion birds need flockmates. Tiels are wonderful birds. I have two males. Generally speaking, they are not demanding.
I have a flight cage for each of my boys. A cage should be at least that big. Natural wood perches are best. If they have only dowel perches, that could lead to foot problems. I feed Harrisons. They get one nutriberry a day and a teaspoon or so of millet. They should not have a strictly seed diet. You can try some fruits and vegetables, but no chocolate, tomato or avacodoes. There is information on this site about dietary needs and experienced tiel people, too. Of course, they should not have junk food or allowed to drink coffee, tea, pop, Koolaid.....those sort of things.
There's a ton of environmental things that are dangerous. Non stick pans can give off fumes we can't smell but will kill birds. No air fresheners or cleaners like Lysol, Pinesol, etc. Blue Dawn is an excellent cleaner. You can always buy Poop-off and/or F-10 if you want a safe retail product for cleaning.
Tiels are dusty. If you have respiratory problems, tiels might not be the bird for you. There's so much I have learned from other tiel people and from this site. Someone else will jump in with more advice and info.
Thank you for not purchasing a bird. My boys are adopted.