Many projects, particularly sweaters, will ask you to pick up stitches to complete a section, such as along the neckline, armholes, or the button band of a cardigan.
Your pattern will generally indicate how many stitches you need to pick up. But that number is based on the row gauge the designer achieved; your row gauge won't always be identical, as it is very common to achieve the stitch gauge but not the row gauge of a pattern. You may have fewer rows or more rows available along which you can pick up the needed stitches.
If too many stitches are picked up, the resulting ribbing (or whatever pattern stitch you're working) will flare out and will not lie flat. If too few stitches are picked up, it will pucker.
You can attempt to pick up the exact number of stitches specified, but you may end up frustrated. Here's how to pick up the correct number, regardless of how many rows there are to work along.
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.
Just remember, most times you’ll be working a pattern stitch that requires a stitch multiple (the likely exception being garter stitch). So be sure you pick up a total number that will work. For example, k2 p2 ribbing requires that you pick up a multiple of four stitches.