As far as I know there is no legislation on this. I think for the Police to get involved it would have to come under 'driving without due care and attention'. I have seen tank transporter convoys of 22 low loaders plus support vehicles, but as I say I don't think there is a 'legal' limit.
When I was in the forces we had to form up in convoys to drive to excercises or for loading on ships, this would involve anything up to 100 + vehicles but we always broke up the convoys into 10-12 vehicles in individual convoys so we did not disrupt other traffic too much.