What changes the cockatiel questions answer
If you keep one cockatiel, you become a major part of its social life. If you keep two, you need space, quarantine, vet care, and a plan for compatibility.
One bird can work
A single cockatiel may thrive with predictable daily interaction, training, out time, and enrichment.
Two birds change care
A pair may focus less on people, may not get along, and may bring hormone or breeding questions if sexes are mixed.
Do not use mirrors as companions
A mirror can create fixation, frustration, or courtship behavior without giving real social care.
Best decision
Choose one or two based on the care you can repeat every day, not guilt.