The Raspberry Pi runs at 5 Volts and a current of 1 Ampere.
The voltage is fixed. For the current: You can run the RPi at lower currents (especially the model A), but to make sure it runs stable, provide at least 1 Ampere. This is true, if you don't connect anything to the USB ports of the RPi. Every device you directly connect to the USB port will draw additional
If you connect more than one RPi to one power supply (like a USB hub with only one power adapter), the voltage does not split up, so it would provide 5 V for every RPi. But the current would split up. You can simply sum all current needs of all the devices, you connect to one power supply. If you connect three RPis to a power supply, it needs to provide 3 Amperes. Every additional device, you connect to the RPis USB port will also sum up.
The 7-Port hub you linked, comes with a power adapter providing 15 Watts. Watts are calculated Amperes * Volts. The power adapter outputs 5 Volts, so you can calculate the available current by dividing the Power (Watts) by Volts. If you divide 15 Watts by 5 Volts, you get 3 Amperes (15 W / 5 V = 3 A). The linked hub would be sufficient to power tree RPis at maximum if you don't connect anything else to the RPi's USB ports.
Current Limit per Port:
The next thing to look for, when buying an USB hub, is how it limits the current on each port. The linked hub can provide 3 Amperes, so it seems like it is no problem to power three RPis connected to three of it's ports. But some hubs are limiting current on each port sometimes even to 0.5 Amperes. For the linked hub, this seems not to be the case. The part about iPads means, the iPad will limit the current, not the hub. If the hub is not intelligent and says "I have a lot of current for you", the iPad will draw only 0.5 Amperes (500 mA).
Power for 6 Raspberry Pis:
To power 6 Raspberry Pis, you would need a power supply that is capable to provide at least 6 Amperes. That is a lot of current. But for example, this USB charger claims to be able to deliver 8 A but it only has five ports. But as it claims to limit each port to 2.4 A, it would be possible to connect two RPis to one port. This would be possible for example with a passive USB hub.
Note on additional USB devices:
Again: If you want to connect additional devices like USB drives, keyboards etc to the USB ports of your RPis, you need their current needs to the whole sum. But as the USB ports of the A and B model are not protected that good, I would recommend to connect devices only via active USB hubs to the RPi, especially if you want to connect USB hard-drives. If you want to connect USB flash drives, they won't add much current needs. Maybe 100 mA maximum. But some of them are drawing dangerous spikes of current for a really short time if you plug them in. I had some flash drives crashing my RPi model B every time I plug them in. With an active (powered) USB hub, that does not happen. Or plug the flash drive in before powering up the pi and never unplug it again.
The Model B+ claims to have better protected USB ports and should be able to provide at least 1 Ampere to connected devices (if the RPi is provided with at least 2 Amperes). This should be enough to power a modern 2.5 inch form factor hard-drive. I did not look at the schematics of model B+, nor tried that, so I cannot verify that information.