So despite having 5 extra platforms than they have coped with for years they won't have enough room, where are all these extra trains using Waterloo coming from?
First problem, the Waterloo international platforms are now only 12 car length.
There are 15 extra 10 car trains worth of capacity to deal with in the peak.
One of the extra platforms was already in use by SWT in the peaks. The next two of the 'extra' platforms are needed because the main suburban side will operate out of 6 platforms rather than 4 in full 10 car mode, because they have redesigned the S&C in the throat.
So all of a sudden it is only two 12 car platforms we need to find a use for, and AIUI they will be used to revise the platforming plan to allow more main line trains to terminate in the high peak hour. More full length trains means more time needed to operate the timetable, and every little helps. Increased platform capacity also allows for normal day to day perturbations and also allows certain trains to spend longer in the terminus before having to be bounced back empty to Clapham Junction...
Somehow I don't think they'd have got the business case together for the rebuild if they weren't justified or intended to be fully used.