Quick maths: 21 guns p/capita divided by 1,000 people = 0.021 * 28 million (pop of Texas) = ~588,000 guns.
Your observation about Texas is very interesting, and I don't have an answer for that. Texas might have, historically, a better gun culture? In any case, having half the mass shootings as another state is still a number greater than zero, which remains the ultimate goal.