A NETCONF client may require that the result is filtered; there are many options for filtering, in your case you may want to use XPath filters in a form like
/files/file[position() > 10000 and position() < 20000]
But: first, it could be that even if the NETCONF client uses such filter, ConfD would still try to retrieve all entries from your data provider. Second, if the NETCONF client requests unfiltered data, ConfD (and the data provider) must provide it no matter what.