Why there is a new WhatsApp channel after I have terminate it?
If you are subscribing to a reserved plan, and your WhatsApp channel to be terminated is the only WhatsApp channel in your app, a new WhatsApp channel will be created automatically after termination as the reserved plan comes with 1 WhatsApp channel. You can redo the embedded signup with the new WhatsApp channel by clicking "Start WABA Setup".
Please refer to the documentation here for more termination details.