The number of nodes is based on each installation. One node per home is typical for an area covered by a transformer. Each transformer can cover a few hundred nodes.
The number of nodes used as a forwarder is a function of routing, which is related by Annex B Routing Cost section on page 117 of the G3-PLC specification (see link below). Thus, depending on channel condition and available nodes, the routing algorithm will choose how many hops to use, and trade off between fewer hops and lower modulation vs more hops but higher modulation (better channel).
Published by the ITU-T as ITU-T G.9903 Narrowband orthogonal frequency division multiplexing power line communication transceivers for G3-PLC networks