How many vertically growing colas do you need to fill the space?
You should work backward from this. Maybe you can do it with 1 topping or maybe you need 2 successive toppings over time?
6-8 is probably fine with 1 topping. more you might want to top successivley making 2 colas from every primary branch off apical meristem (aka trunk).
after that it's just a matter of training to even out the canopy height.
so, your area dicates.. how many plants dictates... how much growth has occured dictates...
you really don't have to think about any of this. just understand how apical dominance works and tr to limit wasted growth.
e.g. if you want 6 colas. don't let it grow out to 8-9+ nodes before topping. I like more than a little nub at top -- first, i don't want to cut too close to the node or you increase chance of it splitting later. second, i like to have a little growth of the axillary buds sprouting form the intended highest 2 branches (just below top cut). so, next node formed and enough internode to cut and leave between 1/4" and 1/2" stem above the node. No idea if the second one matters or not, except for the splitting is a real risk. won't happen everytime you cut it closer, but will eventually if you habitually do it that way.