This epic tacks the adoption of a multi cell triple deployment with 2+ compute cells.
This implies that the source OpenStack has 3 tripleo heat stack

A central stack with no computes nodes hosting cell-0 and an empty cell-1
a dedicated cell-2 stack with computes
a second cell-3 stack with computes

This topology would requiet at least 1 undercloud, 1 controller, 2 cell conductors and 2 comptue nodes one for each of cell-2 and cell-3

in tripleo proper this might force 1 undercloud + 3 ha overcloud controller + 6 conductor 3 per real cell due to triple ha requirements.

there is nothing in nova that would require 13 nodes for this topology
in devstack we can do this with 3 nodes total so one of the challenges is how to test this in ci without need a very large number of nodes.