Instinctively, based on MTB tires (because these are the same width as MTB tires but a smaller diameter), I would have said "2, maybe a shade more, but 2 for what I run my tires at".
From a math perspective, I'd have guessed 2 as well. PV=nRT, the ideal gas law. nRT is constant if comparing two tires at the same temperature, inflated from 2 CO2 cartridges. P is lower for 29er tires by 60%, V is lower for CT125 by 60%, so those two cancel each other out. They sell 38g threaded CO2 cartridges to get those large volume tires up to ~20 psi, or tell you to use 2 and a pump.
Fun fact: 3.75" tires were the maximum size a bicycle tire could be made at before Surly Bicycles (subsidiary of QBP) went to Taiwan and reconfigured the tire manufacturing machines so they could produce fat bikes. Now they make up to 4.8" tires.