For sky wave propagation of a `10 MHz` signal, what should be the minimum electron density in ionosphere?

For sky wave propagation of a 18 MHz signal, what should be the minimum electron density in ionosphere ?

For sky wave propagation of a 15 MHz signal what should be the minimum electron density in ionosphere ?

If the critical frequency for sky wave propagation is 12 MHz , then the maximum electron density in the ionosphere is