There are too many factors to consider to generically assign a curve number to a wetlands, including the location in the watershed, presence of groundwater, topography, outlet hydraulics, and potential to impound water and attenuate flows. In most cases wetlands should be modeled as a level pool with the outlet modeled as accurately as possible. In this case the open water body would be CN 100.
Curve numbers could be justified for a saturated slope, for instance, that would generate sheet flow runoff and not impound water, in which case Soil Conservation Service Mapping - Hydrologic Group D curve numbers could be applied. Your local agency or conservation district should have a curve number chart for your area.
