Depends on the color and the timeline.
If you have a solid color- one that doesn’t have any flake or pearl in it- you can buy enough to paint the whole car at once. Your timeline is the shelf life of the paint. If you do that, you can paint one piece at a time and should have a close enough match between the panels to not have obvious discrepancies. But the paint has to be well mixed every time, and the original paint needs to be out of the same batch. You have to keep all the mixing proportions and gun setting the same throughout. You can definitely screw it up, but if you’re careful it can work. It also will depend on the paint and the color, some are easier than others.
If the paint you want has any metallic aspect to it it will have flake in it. The problem with flake is that it will make the same color paint look a different color depending on the amount of flake, the depth at which it’s distributed in the paint, etc, etc. Temperature and air pressure can change the color. And if the paint isn’t perfectly mixed every time you’re done. I have seen paint with metallic flake out of the same can sprayed on different days look like they were almost different colors. Same with pearls. Basically, if your paint has any flake or pearl in it you have to shoot the whole car at once. Otherwise your chances of matching it are pretty slim. I won’t say impossible, but darn close. Easier to screw up than to get right for sure.