I agree, although I don't have a particular number in mind.
I also think there should be a few other tests of basic mental health and intellectual competence. They don't need to be demanding. Just adequate to test if a person looks likely to accept election results or mount a coup, and knows which side he is on between the US and North Korea or the US and Russia or the western democracies and the dictatorships. Those sorts of things. Low bar, I used to say.