From PRI experience, I can tell you this channels are allocated dynamically. And I want to say the same will likely apply for an E1.
If you were to think about it really. The number of available channels will not necessarily equate the available phone numbers. Say for instance your carrier allocates 100 TNs or even maybe just 20 TNs to you. Either ways all the available chanels on your media (E1/T1) will be availble to you except you get something like a fractional T1/E1 but still you will be able to get calls for any of the numbers since the provider will send it to your available connection. The channels are just transport mediums for your 64k calls. You wouldn't buy/get 4 E1 connections to get your 100 numbers routed to you.
Moreover that is why it is digital connection. It is not circuit switched.
You can even test it yourself. Go to your voice gateway (assuming it a cisco router) and do "
sh voice call sum" and watch your channels. Make a call to a number. Watch it use a channel. Then make another external call to the same number and pick up or simply watch again from the command. You will see it use another channel.