it is not exactly mean that if u have 1 trx then u have 8 simultaneouslly 8 speech calls.
it depends on ur control channel configuration.
if it is combined then u have 1 timeslot for control channel and other 7 is for voice and if ur conf. is non combined then ur 1st and 2 nd will be used for control channle while other 6 will be used for voice channel.
ask if any doubt