An E1 by definition is 2.048 Mb, commonly referred to as 2 mega-bit bandwidth.
In reality it is divided into 32 channels (30 voice, 2 control)of 64kb each (32 x 64kb=2.048Mb).
64kb is the standard used by telcos for g.711 voice encoding. If you use this encoding you have 30 voice channels and therefor the possibility of 30 similtaneous calls.
It is possile to utilise different encoding and achieve more voice channels per link.
Note that the concept of seperate channels is only one way of utilising the bandwidth.