Fan out in DC convertors
The term "fan-out" doesn't really apply to power supplies. The procedure you need is straightforward. I assume that you know how much current your DC-to-DC convertor can produce, so now you need to add up the maximum current of each IC that is powered by that convertor, and insure that it doesn't exceed your convertors capability.

"Fan-out" originally applied to logic gates in the old 7400 series, before 74S and 74LS made things more complicated.

With all of the different semiconductor technologies used today, and because output drives and input currents can easily be tailored to the application, output drive current and input current consumption vary wildly. To know if your output has enough drive, you need to add up the required input currents and compare it with the output current of your source. You would need to do that for both source and sink (high and low) from the information in the datasheets.

Today, "fan-out" is mainly used with respect to the gates inside an FPGA or ASIC.