There are certain high-end paints that only require one coat (quality paint is worth the extra cost), but most paint you get at Home Depot, or Lowes requires two or three coats, sometimes more, depending on the color.
Sometimes you think you can handle projects yourself, but it might actually be too much. Spend a little time on YouTube watching others complete a project you’re thinking about taking on.
Unless you’re planning to use an all-in-one paint and primer, you should always use a primer before tackling a painting project. And, even if using an all-in-one primer, you often will get better results with using a dedicated primer, then paint.
Project budgets have a habit of increasing over time. Something goes wrong, different materials need to be purchased, or not enough materials were purchased.