there is no theoretical rule to define the ration SCCH / TCH, it all depends on your subscriber behaviour (amount of SMS especially). The rule of thumb is : 8 SDCCH for 14 TCH.
In some countries, the amount of SMS is so high that you need 3 or 4 SDCCH timeslot (24 to 32 SDCCH) in order to cater for the SMS trafic.
Anyway, you can just start your cell with 4 SDCCH and check the sDCCH congestion. Then, accordingly adjust the number of SDCCH if you measure SDCCH congestion…