How to calculate required amperage of multiple motors?
I don't know if the amperage is additive with each additional motor

Yes, amperage is additive.

the Arduino cannot handle over one amp but 0.24*5&gt;1

That is correct. But the stepper motors should never be powered through the Arduino. You should connect a fitting power supply directly to the stepper drivers. That way the needed current flows past the Arduino, not through it.

What voltage and amperage I need to supply to my Arduino for everything to run properly?

The Arduino runs at 5V, though you can provide 7-12V to Vin/barrel jack. When your motors are rated for 5V, you should provide them with 5V. So it would be fitting to buy one 5V power source to power the Arduino (through the 5V pin or USB) and the stepper drivers.
For the current: A good measure is the stall current of a motor, so the current, that flows, when the motor is driven, but cannot move (due to being physically stuck). Add a bit headroom to this. The mentioned motors are often badly documented, so that you might not be able to find a stall current value. Add more headroom in that case. So I would suggest adding up all the amps and then multiply this value with 1.5 or even 2. Let's take 250mA per motor (for ease of calculation) with 5 motors gives 1.25A. Take at least a 2A power supply. When you want to make sure go directly to 3A. The price difference in that range isn't that much anyway.