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Presumably you mean for the filament heater in a valve/tube?.
I don't see that they vary at all?, except they tend to be unusual voltages (6.3V is the most common), and of course they normally have a constant load - so you should design it to give the correct voltage under the load it's designed for.
But to be honest, the slight variations caused by using it with 2 valves or 5 valves, isn't going to make a great deal of difference to the voltage.
If I was building something today, I'd most probably just order a standard 6V transformer to feed the heaters.
You might also consider feeding them using DC?, you could regulate it then, and it would help to reduce mains hum in an audio amplifier.