Currently, the number of items corresponding to the expansion of a symbolic group name is not communicated by the Server to the client.
We always assume that the client knows in advance the meaning of the symbolic group names it submits to the Server,
so that it has already been able to provide the suitable space on the page for displaying the value before subscribing.

For any case of management of lists of unknown length, the COMMAND mode and the DynaMetapushTable were provided.
Note that the DynaScrollTable, combined with DISTINCT, mode also displays variable length lists, but it doesn't allow you to update the values, only to grow the list.

Otherwise, application level mechanisms have to be leveraged; for instance, you could subscribe to an item which provides the list length.