There are 6 dc - dc buck convertors in this project, while not as inefficient as linear regulators they can still get toasty. If the supplies are only getting light use the there is probably no need for a fan, just make sure that you have the base raised (using stick on rubber feet) as there are some holes underneath for air flow.
If you are ramping up the amps then a bit of assisted airflow is worth considering. I have included mounting holes for a 60mm x 60mm x 15mm fan on the inside of the main unit. I have a few of these on order from AliExpress to try it out but not needed it yet.
If you have the full unit with the power supply then the fan will pull air though the main unit and push it through the PSU so the one fan should do the job.
The LM2596S Dual USB Power Supply Module is fairly cheap (~$1.5) and can churn out 3A (2A + 1A). If they are being pushed above 2A adding a small heat sink would be recommended (only cost a few cents but I had a couple spare from A4988 driver boards)