1 usb and 1 mmi. Not tested any splitters yet but Audi do an MMI to usb socket type cable so you can run 2 usb devices in terms of power. I keep meaning to buy one of those cables that has 3 wires running off it for usb and iPhone and lightening connector. I'll find a link
All UK A3's come with AMI - and you'll get (on request from dealer) 2 AMI cables. If you have an Apple iPhone you'll need a yellow collared cable with Apple Lightning interface for iPhone 5/6, and a USB socket cable. These cables plug into the AMI socket in the cubby box, and you can plug in your phone or HDD to charge, stream music via cable etc.
(Yes, you can also stream music via Bluetooth or WiFi, depending on whether you order Tech Pack etc.)
However - if you order the Audi Phone Box or Tech Pack with Connect
You will get, in addition to the AMI socket in the cubby box, a USB socket, that is used solely for charging.
I notice from reading this thread that you get different colour cables for different iphones but what if you own a Samsung phone that charges via a micro usb ? is there a certain cable to ask for for this phone ?
Worth mentioning that I'm pretty sure the USB socket charges at a much faster rate than the AMI cables, and also allow you to have a music source connected to the AMI that isn't your phone while still charging.
There is a micro USB AMI cable (I have it) which you can ask for, but it doesn't come as standard.
And as others have said, the tech pack DOES NOT come with the USB socket unless you also spec Audi Connect.