I can get it to 150 units (36x150, 26x153, 24x158, 1x170, 40x197, 5x230, 11x231, 7xMk4) but struggling with the last one unless another 197 has been accepted.
Of these there are 17 stopped for at least the last 2 weeks. 150217/229/242/254/260/279
153909
158834
197001/007/009/012/014/048
230006/230010
HD03
These are waiting parts/collision damage/wheel damage etc mostly out of the ordinary pattern.
The majority of the others would be on routine maintenance. The original ATW fleet would have had units stopped too as will all fleets so it doesn't seem untruthful to say that the full figure is the amount in the fleet.
As
@Caaardiff said there are 110 diagrams currently on a weekday so that leaves 23 units on routine maintenance/exams/short term stopped before availability becomes a problem.