This is a complicated question with a series of complicated answers. Here’s the high points:
Most people - even most hospitalized patients - don’t need a red blood cell transfusion (i.e. “need blood”). For that matter, most folks who undergo a surgical procedure don’t need blood (those surgeries that do require blood most often use small amounts - 1 or 2 units (300 or 600 mls). Acute medical conditions (GI bleeds, some hemolytic anemias) usually need small amounts of red cells (though there certainly are exceptions).
While the above is true, a small handful of planned surgeries can use 10+ red cell units (~3L) usually because of unrecognized bleeding disorders or complex/difficult intraoperative issues. A few kinds of surgeries can end up needing 20-30 units (liver transplants are notoriously bloody ) - 50 or even 100 unit surgeries do happen on occasion.
Trauma surgeries can use lots of red cell units, but more than 10-20 units is unusual (trauma cases tend to use more non-red cell blood products like platelets, plasma, cryo, factor concentrates, etc)
Okay, so if most people don’t need blood and most surgeries don’t use much blood, where does it all go?
Cancer patients need a lot. Plus a collection of other (mostly chronic) conditions that either lead to poor red cell production or overly rapid red cell consumption (see #5).
Cancer patients can need frequent transfusions (from a unit or two a day to a couple of times a month) for extended periods of time - using a hundred units a year is not unheard of (though I’ve seen this more with platelets post- bone marrow transplant).
Speaking of bone marrow transplants, most post-BMT patients need red cells for a few weeks post-transplant…and a lot more BMTs are being done these days.
Chronic conditions can lead to short term, low volume use (liver cirrhosis, GI errosions, etc) to long-term, medium-to-high volume use (monthly 10+ unit prophylatic red cell exchange transfusions for sickle-cell patients).
“Wrong” blood. In addition to the ABO and Rh blood types (where “A positive” or “O negative” come from), there are hundreds of other blood typing factors - at least two dozen of which are clinically relevant at some level. The point is that getting blood for a given patient can range from “almost any unit will do” to “Call the rare blood type bank in Philly - maybe they can match something”. Thus, you may have 200 units on the shelf but still need to order blood.
Waste/loss. As noted previously, red cells have a short lifespan even when frozen (and there’s decent evidence that “old” cells are physiologically less effective than “new” cells). Also, while your red cells have been lifespan of ~100 days, transfused units are more fragile band only last ~50 days…kind of a transfuson treadmill of sorts.
Blood banks do an excellent job of shifting products around to try and avoid “outdating” (at least in the US), but some blood does get wasted. Good banks try to keep this number well under 5% (though this target is variable depending on…lots…of factors).
- Everything else that uses lots of blood products. While I’ve focused on the red cells (since that’s what the question was about), plasma and platelet concentrates are also blood components that come directly from humans and are heavily used - more than red cells these days.
Technology also changes usage patterns: cardiac assist devices (LVADs, ECMO, etc) save people who come into the hospital “mostly dead”, but they eat blood products at a crazy rate. On the flip side, increasing laproscopic/robotic techniques reduce surgical blood use substantially.
I’ve left out lots of stuff, but this should cover the high points.