There is no minimum quantity for special design couplings.  We often design and build special designs for just 1-piece, but can also support custom designs that require 100s or 1000s of pieces.  We are happy to discuss your application to understand the requirements and provide a custom design to fit the needs of the application.  In almost all cases, there are no upfront Engineering fees involved.  Please call to discuss if you need a special design for your application.