There are many times you'll need to pick up and work stitches to complete a project: along the neckline, armholes, or the button band of a cardigan are just a few examples.
Your knitting pattern usually tells you how to many stitches to pick up and where. But that number was based on the row gauge the designer achieved; your row gauge won't always be identical. You may have fewer rows or more rows along which to pick up those stitches.
If there are too many stitches picked up, the resulting ribbing (or whatever pattern stitch you're working) flares out and doesn't lie flat. If too few stitches are picked up, it will pucker.
You can try to pick up the exact number of stitches specified but you may end up frustrated.
When you pick up stitches along a vertical or curved edge, pick up one stitch every four spaces (the space you insert your needle into in order to pick up the stitch). Skip the next space and pick up the next sequence of four stitches along the next four spaces. If you are using a finer weight yarn at a gauge of about 28 rows per 4 inches, try picking up five stitches per sequence instead.
Along a horizontal edge, always pick up stitch for stitch.
One word of caution. If you're using a pattern stitch that requires a stitch multiple however, be sure to pick up a total number that will work. A k2, p2 ribbing, for example, requires you pick up a multiple of four stitches.