Fractures in the pelvis can cause from minor blood loss of about half a litre to major injuries where you can loose all your blood. The main femoral artery runs over the pelvis which is a major artery.
Injuries to the spleen, liver or chest can cause very minimal blood loss to total blood loss.
The big problem is recognising that severe blood loss is possible or actually happening. It’s best to assume the worst and treat as if there is sever blood loss. It is very hard to work out how much blood has been lost with an external bleed and even harder with an internal bleed. It is also difficult to assess blood loss as different people will react in different ways which can cover up signs and symptoms until it gets very serious. Things that can affect the way signs and symptoms show are age, size, weight, under lying medical conditions, their fitness and any medications they are taking.
You need to get as much information as possible about the history of what happened and the assess the mechanism of the injury.