If you're considering simply active forum-goers, then probably well over 1,000. However, since we're not actually at the stage yet to have costuming members officially approved and deployed as 'active', we don't have any official 'active' numbers.
With that said, I'd estimate that we have perhaps 500 fully-finished, ready-to-deploy costumers, regardless of costume quality. Again, those figures don't take into account those that have completed costumes and then gone inactive.