If we burn all carbon in the Earth's crust, by how much would the atmospheric concentration of $CO_2$ increase?
Short answer: $\rm CO_2$ levels would increase by $209,460$ $\rm ppm$ by using all the available oxygen.

Long answer: The mass of carbon in Earth's crust is $9(10)^{22}$ $\rm gC$ (grams of Carbon). The mass of the atmosphere is $5.15(10)^{21}$ $\rm g$. If all the carbon was burned, or otherwise moved to the atmosphere, then the limiting factor would be the $\rm O_2$ (molecular oxygen) available for carbon to bond with to make $\rm CO_2$. Oxygen makes up $209,460$ $\rm ppm$ of the atmosphere. So, assuming the carbon bonded with all available $\rm O_2$ and the remaining carbon did not enter the atmosphere, then there would be an additional $209,460$ $\rm ppm$ of carbon dioxide in the atmosphere.