I hope you are not confusing hosting domains with registering domains.
You can host unlimited amount of domains on DH, but every domain that you register after the free one that you get, you have to pay for it.
Also if you have any other domain on another registar, you can either set the Name Servers to DH for free. But if you transfer the registration to DH, the next rebills of the domain will be done by DH, and not your old registar.
Hope I helped.
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