It is generally recommended to replace all four of your car's tyres at the same time, even if only one is worn out. Replacing all four tyres ensures that they are uniform in terms of tread depth, type, and age, which can affect the vehicle's performance, handling, and safety.
Replacing all four tyres may be more expensive in the short term, but it can save you money in the long run by extending the life of your tyres and reducing the risk of having to replace them prematurely.