The Raspberry Pi model B comes standard with 4 external USB ports. Although this is so, Pi has only one root USB port, which funnels all traffic from all connected devices down this bus. It operates at a maximum speed of 480mbps (USB 2.0). Apart from this, this post also gives a little more about the Raspberry Pi’s USB ports.
USB specification defines three device speeds – low-speed (USB 1.1 – 1.5mbps), full-speed (USB 1.1 – 12mbps) and high-speed (USB 2.0). The Raspberry Pi default is high-speed.
It is said that there are generally no issues with connecting multiple high-speed USB devices at the same time, but even my own experience tells me that there are soft limitations on the number of simultaneously active low- and full-speed devices.
Changing the port speed
In some cases one might need to change the USB port speed for older hardware to full-speed (i.e. USB 1.1) use:
sudo nano /boot/cmdline.txt
and appending the following:
to the very end of the line.
Another important thing about the Raspberry Pi USB ports is that they will only happily supply 100 mA per port. This means that “low-power” devices like USB drives, keyboards and mice will work sufficiently, but if you start to get funny behaviour, like devices stopping without a reason, brownouts and/or unexpected resets, you should probably get an external USB hub with its own power supply.
Devices such as WiFi adapters, USB hard drives and USB pen drives are known to consume much more than 100mA. It is also noteworthy that in order to supply even low-power devices with sufficient current, the Pi should also be supplied with a proper power supply. The people at the Raspberry foundation recommends 5V with at least 1.2A (1200mA). The official Raspberry Pi power supply from RS-Online will give you 2A!
Effect of overclocking
I have heard somewhere that when the Raspberry Pi is overclocked, it will supply less amperage.