sdcch are use for the call setup, a SDCCH is released as soon as the call is put on the TCH channel.
So in your case :
Full rate, 1 TRX (7 TCH, 1 C-BCCH) can carry 7 calls at once
Half rate, 1 TRX (7 TCH, 1 C-BCCH) can carry 14 calls at once.
Keep in mind that you should rather use the theoretical capacity of the cell (erlang B law) when computing the voice traffic a given cell can carry.