What consitutes a MCI would vary from place to place. In my county, it would not take much to create one. With a population of 20,000 in an area of 452 square miles served typically by 2 ALS units, the number of patients can exceed the number of rescuers easily. Backup units from neighboring counties depends upon availability with response times of over 20 minutes. We can get helicopters faster than that. Closest hospitals are 30 minutes away, since we have none in our county. Level 1 trauma centers are over an hour away by ground under optimum conditions. Few of the few first responders in the county are EMT-B, the rest are just there to help.
The last MCI consisted of 5 patients from a wreck. Four of them were over 70 years of age, with injuries and mechanism of injuries that dictate evacuation by air. For example, symptomatic head injury, extremity amputation, unstable pelvis. We could get only two single patient helicopters to respond. Usually, there are three single patient helicopters and a dual patient helicopter available. Given the time of day and traffic conditions in the major cities where the level one trauma centers are located, two of these patients went by ground to a level II center due to its proximity. The fifth patient had a fractured tib and went with the others.
To handle this call, one ambulance was pulled off of a call with a low priority patient, who was given the choice of going to the hospital via the wreck or not. Another ALS unit responded to the call originally. Another unit was staffed by an off-duty paramedic called from home and a volunteer driver.
The call was in a part of the county with a time to scene of 15 minutes on average. All in all, it went smoothly. The last patient was off the scene within 25 minutes of the arrival of the first unit. The first unit took two patients to the LZ, the other two trucks came from about 15 miles away. Even if the the third truck had not been available, the first unit was on a time schedule that would have allowed it to return to the scene from the LZ for the last patient in the same time frame as the arrival of the third unit. All the patients were gone before law enforcement arrived.
I have run a MCI before where some of the critical patients were left on the scene with a trooper until the sole unit could return to take them. That is tough triage, deciding who will go first and leaving the others until you return.
Rural EMS is different.