1 Erlang is a circuit being used for a duration of one hour.
The SDCCH capacity is affected by the configuration you are using. For example it will differ if you use combined or non-combined SDCCH, or if you have additional dedicated SDCCH channels, certain radio feature.
Since SDCCH is a set of sub-channels, the total capacity will consider the number of sub-channels as the number of circuits and QoS requirement should be more aggressive, typically 0.5%, now you can use both figures to calculate the supported traffic from teh Erlang B table.