A blood donation truly is a “gift of life” that a healthy individual can give to others in their community who are sick or injured. In one hour’s time, a person can donate one unit of blood that can be separated into four individual components that could help save multiple lives.

From one unit of blood, red blood cells can be extracted for use in trauma or surgical patients. Plasma, the liquid part of blood, is administered to patients with clotting problems. The third component of blood, platelets, clot the blood when cuts or other open wounds occur, and are often used in cancer and transplant patients example:
Red Blood Cells: anemia
Platelets: leukemia, cancer
Plasma: blood clotting disorders
Cryoprecipitate: hemophilia

Medical advances and modern surgical techniques, such as cancer treatments, organ transplants, and open heart surgery, have increased the need for blood. Depending on patients' needs, your blood may be separated into several different components, which may be used to treat a variety of medical conditions or illnesses.

You've got about 5 billion red blood cells in every liter of blood (that's 5 followed by 9 zeroes, so all in all, it's quite a lot). Which means you have on average 25 billion red cells running around inside you, although 25 million (or 0.1%) of them die every day. That works out at roughly 300 a second, so it's quite unlikely you're going to run out of them.