The plan with the Pacer replacement as I understand it, is that 9 of the 30 will be directly replaced by the 9 769s, and the introduction of the 12 170s and 5 230s on the wider network will facilitate an "internal" cascade of 17 150s to "Valley Lines" (pretend it's still a seperate TOC) which will mean that "their" allocation will go from 15 142s / 15 143s / 6 150s, to 23 150s / 9 769s. So 4 fewer units, but a horizontal carriage space increase on that network/sub-network of 500 metres from 1140m to 1640m.
Per unit, Pacers have 2 15m carriages, 150s have 2 20m carriages and 769s have 4 20m carriages. And the Pacers have to be doubled up to achieve still only 75% of the length of a single 769, so the 4 fewer units won't be an issue. 9 769s are of equivalent capacity to 18 150s, not 18 Pacers. 9 769s are of equivalent capacity to 24 Pacers, due to Pacer carriages being only 15m in length rather than 20m.