To fit properly into a membrane a protein must have one or more hydrophobic sections, each about 20-25 amino acids long. The reason for this number is that cell membranes are rather thin, and 20-25 amino acids is enough to go across from one side to the other.
Many membrane proteins cross the membrane multiple times.
This glucose transport molecule, for example, crosses 12 times. There are 12 hydrophobic sections imbedded in the membrane. The figure is a rough diagram of an extremely important membrane protein. It represents GLUT4, the glucose transporter that is sensitive to insulin. It is found in the muscle and fat cells of your body.