Each manufacturer varies it's way of calculating what is required. Also for the NTC - yes it is initially but what you do is then short the NTC using a relay. If the power goes, the relay cuts and the power restart goes through the NTC.
Never parallel NTCs. You can't guarantee the load is split as you'd expect.
Serial NTCs can be used. However there's an argument for using a larger NTC as the larger disk cools.
Typically an NTC is useful for small inrush, beyond that you need to design in an inrush strategy for every eventuality into the amp.
I'm attempting to keep under about 5A, so I've split my heater and B+/B- rail supply starts. For a normal tube amp that's not a problem but for an OTL with 16 tubes and 9 power supplies you can't simply kick the whole lot off an pray not for a melt down.
The heater supplies (RC filtered) kick off first, that's about 111W once the caps have charged, a NTC kicks in to start warming from their dead short start the heaters then switches out. That PSU is small enough not to need much of an inrush.
The other power supplies are larger but lower current so apart from the transformer and chokes, the only PITA is the +320V/-320V that has some larger decoupling caps. Too larger and you start seeing inrush. Too small and you loose the filtering.
Some (rare) types are specifically designed for that kind of pulse operation, and have some kind of relevant spec, like I²*time for fusing, but in practice, all models having the resistive wire in direct contact with a cement, a vitrified covering or a silicone moulding compound withstand a few joules of pulse energy, meaning most regular WW resistors can work