Other, longer, workings in the area were the return serviced two car sets from Newton Heath to Preston to be split there to work the local services during the day. I suppose every engine and axle light illuminated on the driver's panel would be the limit. Not sure how many units that equates to.
It won't win any prizes on this thread, but in similar vein, during the 1970s there was a DMU around 19:00 or 19:30 each evening from Southport to Manchester Victoria which regularly ran as 7, 8 or sometimes 9 cars.
This wasn't a "bucket & spade buster" - rather it was a lash-up of several afternoon peak Manchester to Southport trains returning to Newtown or Cheetham Hill carriage sidings, which ran in public service to Man Vic. As in
@furnessvale's case, probably all 6 sets of lights illuminated on the driver's panel - given the propensity for deploying D3 Power Twin units around the Manchester area.
The front couple of units were kept locked - passengers having to use just the rear-most unit where the guard would be travelling.
For a time I was a regular user of this train. The first time I caught it, it was quite disconcerting to be waiting on the platform and seeing the lengthy train seeming to sweep through my local station as if it wasn't stopping, before pulling up with the rear cars positioned at the normal passenger boarding area.
Most annoyingly to me, you would probably have two (relatively) comfortable Class 104 "White Stripe" units locked up and in darkness, whilst everyone was forced to travel in a rattling, vibrating 2-car Cravens 105 at the back.