for SDCCH : check out your vendors recommentation : for a certain number of TRX in a cell, he will tell you how many SDCCH are necessary.
Default usage, one SDCCH/8 will cater for 2 TRX.
If you have a lot of SMS’s, you might need some extra SDCCH channels, or some dynamic SDCCH/TCH timeslots.
For AGCH, you can choose one block minimum + dynamic allocation of AGCH blocks over Paging blocks.
In both cases, it is always possible to check the QOS (congestion SDCCH ? congestion AGCH ?) in order to reconfigure the cell later on.