C2DM quotas are indeed based both on the number of messages for the device and on messages sent to any device.
However, it seems that nowhere is it indicated what the amount is for one device.
When you register, you give an estimate of how many messages you expect from sending your application. (And a standard quota of 200,000 is assigned)
You can read about c2dm limitations here
And you can read about quotas here . Here you can also request more than 200,000 messages by default.