There isn’t an explicit equation, but it is simple to figure out from the information provided in the datasheet:
The LO frequency will be N.F * fref where N.F is the PLL feedback division ratio (N is integer part and F is fractional part of this number) and fref is the reference clock frequency (i.e. the crystal frequency).
The fractional part of the division ratio has a resolution of 20 bits. So the minimum step is 2^(-20) * fref.
As an example, on the MAX2223 EVKITs, we happen to use a 27MHz crystal, so the minimum step is 27e6 / (2^20) = 26 Hz.
The minimum allowable crystal frequency is 12MHz, so the minimum possible step is 11 Hz.