How many flash drives can you power with an iPhone battery?
Apple doesn't say outright how much power you can draw via a lightning to usb host cable or adapter but its about 200 mA before the phone alerts you. So unless you have an externally powered hub this isn't working at all.
The current ratings for your devices should be the maximum they draw. That may be the operating current or spikes. It may also be what they request from the usb host, as the usb drives should be asking for permission to draw a lot. When in sleep mode, like when a flash drive is not being read or written to, the draw will be lower. Reading is also less power hungry than writing. So its hard to say what your power draw is with specifics. If we assume full power, plus conversion from 3.x to 5V of 80% efficiency it would drain the battery quickly at over 1A draw for a single drive.
Since everything is in parallel power wise they all get 5V and share the available current. Since they vastly outpower the available current supply that can lead to over drawing and damaging the source aka the iPhone. Luckily there is power limiting inside. So nothing will work but you may still damage the iPhone without a powered hub.
Finally no you can't just provide the equivalent wattage to a device. They will need 5V 900mA (or so) to work. Giving it 2V 2.3A wouldn't work. Current is drawn not pushed.