Sometimes budgies do well as a single bird, even after being accustomed to having other budgies around. Some do, some don’t. I wouldn’t necessarily have the mindset that you “have to” get another one when one eventually passes.
Usually budgies take a couple or few weeks to adjust to the new norm after losing a friend. After about a month or two, it will be more clear if a single budgie is an individual who can be happy as a single bird.