The fact no one can have a good answer as to why not 6 or 8 does not mean the law limiting it to 7 is bad law. Does it?
it just means it's arbitrary.
Do you like arbitrary laws?
To me it indicates that the people crafting the law have no idea what they are regulating or why - if they could produce some sort of data/study/etc showing that 7 is the "magic number" that tends (correlation) to decrease (if not directly impacts, causation) the deadliness of a mass shooting, then I'd bite.
Laws need not be perfect
, but should at least be rational.