Acutally, that's explainable. The series and Endgame explain that their are thousands of Immortals. Joe shows Duncan a head count of each Immortal Duncan has killed. 174 at the start of the film. Connor has killed at least 280 something, I think. Jacob Kell, the bad guy has killed 661 after Duncan's first meeting with him early on. In other words, there have been at least 1 to 2 million Immortals all told. But I lean towards 1 million. The second film said that Immortals were aliens from Zeist sent to Earth as punishment for war crimes. But the fifth film will give us the true answers. The second film has been completely ignored as it should be.