Not sure that surge is as much a problem with SMPS as with a linear supply as the storage capacitors are much smaller, but there will be some. Frankly, it's all a bit finger in the air but for what it's worth, I have a machine with 4 steppers. drivers set to deliver around 4A each at a nominal 70V. That's about 560W as I assume that only two steppers are taking full current at any one time. As I say, there's a fair bit of guesswork in all this! The rest of the control box (motion controller, relays, etc) take so little that they aren't worth considering. However, I have fitted a 10A D curve MCB. 10A gives headroom to allow for switch-on surge for the linear supply, and the D curve is there as it has a slower response time to help with this.I am confident that even with a major fault internally the MCB will trip before the wiring bursts into flames - and, frankly, that's the only protection that the MCB gives.

One MCB for the whole box - I don't feel that there is much point in a separate MCB per driver as some people do. At least my way if the MCB trips then everything is going to stop rather than just one axis. But.- you pays your money and you takes your choice.