In general when using SCROG there is no exact height. Maximum height depends on lights used (and their ability to reach bottom).
Don’t fix to particular number of plants. All that matters is timing of 12/12 switch and you won’t get it exactly right without knowing your strain well. Generally you want to fill screen horizontally at least 70-90% and then switch. You will use stretch spurt to fill rest of the canopy horizontally and let new shoots protrude up and form number of colas.