T1 connections have 24 channels in total.
Typically, for T1 ISDN (CCS) protocols, 23 channels are used as B-channels and 1 channel is used as a D-channel. For this reason, the frame structure of a T1 PRI connection is said to be 23B+1D where the D-channel is located on timeslot 23 (channels numbered from 0 to 23) and the remaining channels are used as B-channels. Hence;
Channel 0-22 - B-Channel (Voice)
Channel 23 - D-Channel (Signalling)
For T1 CAS protocols (for which there is only one; T1 Robbed Bit), all 24 channels are used as B-channels.
For information on E1 connections please look at the knowledgebase entry 'How many voice channels does an E1 Primary Rate Interface have?'