How should I determine the switching frequency of LT8622S?

Hi miky,

miky said:

how many switching frequencies I should use

Are you asking for the best switching frequency to use?
It depends on your priorities. Higher frequencies minimize size of inductor and capacitors and maybe also lower output ripple,
but tend to be less efficient. 
Very important is how much output ripple voltage you can tolerate?
And the converter is able to operate under different modes, depending on load.
Pulse skipping or burst mode is efficient at light load, but the lowered frequency will increase ripple.
So for minimum EMI and ripple, design your inductance and switching frequency,
so that with your load current, the converter is in continous mode.
I would suggest, that you simulate your circuit in LTSPICE. 

At least, I would suggest a switching frequency of 1...2 MHz and a high enough inductance to enable continous mode for most of your load current.
And in data sheet on page 24 there is a discussion about switching frequency.