According to Sam Sung’s questions I should clarify few things as well. First, you are right: the spacing in DWDM and CWDM systems is standardized mainly for given bit rate (not sure which bit rate you are planning to use for 12GHz). The increment of the channels number will bring several limitations with it. The main of them are nonlinear effects influence. Hence, these effects needs to be considered in system performance not just by changing the fiber but by using corresponding compensations!