Zero. Mass doesn't exist. It's a manmade concept to create equations. Without the mass fudge factor, equations wouldn't equate.
The fact it is a manmade concept doesn't override it's validity or existence. If it did the same could be said of everything in existence that has a human-given nomenclature. Width, depth, dark, light, cold, heat. All physically exist, but are they fake because humans gave them names? Heat and cold are nothing but different states of molecular excitation.
Light and dark the presence and absence of protons moving at or beyond a certain speed.
Length, width, depth, all measures of the three dimensions most commonly perceived by human beings as a species.
It comes down to how much detail you are willing to put into every sentence. Instead of trying and failing to convey a message with near cryptic complexity in some masturbatory display of my intelligence that more accurately depicts my point, I'd rather use the established vernacular system and let the layperson believe they actually understand me.