I’m not sure how many authorities insist upon in order to be classed as a serial killer.
And I don’t know that it’s a matter of numbers at all in it’s essence. A murderer and a serial killer could have the exact same totals ( whether that be few or many) and it doesn’t speak to the main issue germane to the classification.
There are many different types of people who, given the right
set of precipitating circumstances, would be capable of murder. But it takes a distinct type of mind to repeatedly plan and carry out the cold blooded killing of others time and time again. Someone totally devoid of empathy or conscience who even takes pride in these acts, essentially a sociopath. I think this is what prompted law enforcement to create this category and ” serial killer” is what they agreed upon as a common name.
They had to describe this category in some way because most other murders are committed by someone known to the victim ( not every single one but def the vast majority) so there is a methodology to determining who it would be.
With a serial killer, all that goes out the window and a different approach is called for.
So, I would presume that the exact number is less important to the classification.