What was the most units that were ever required from the fleet of 76 before these reductions and why was the demand so much higher. I rarely use the Wirral line but all I hear about it is that 6 cars would be overkill on most services especially with the higher frequencies they get over there.
The decline in capacity affected the Wirral Line more than the Northern in the 1991-94 period, hence my mentioning it. The Ormskirk line did lose at least one of its six car workings each peak time in this period too: it used to have four out of five formations paired. Otherwise, the Northern Line kept its six cars and 7/8 minute Southport line peak service. I don't know the highest number of required 507/8s, but it can't have been much more than 60.
When the Link and Loop tunnels opened in 1977, the Wirral Line (19,000 passengers travelling into Liverpool Central on the average weekday) was busier than the Northern Line (14,000). This was partly due to the inconvenient location of Liverpool Exchange for the shopping and nightlife areas of town, hampering off-peak travel. The Northern Line also only had two electrified routes at the time, the Southport and Ormskirk branches, although the Southport line was the most heavily used individual route on either side of the Mersey - and still is.
Since then, there has been a complete switch around: the Northern Line overtook the Wirral in 1983 and the gap has widened since then leaving the Northern Line much the busier of the two. Reasons include the opening of Liverpool Central's Northern Line platform and the incorporation of the Kirkby and Hunts Cross routes, as well as a gradual decline in Wirral Line patronage. In 1987, the average weekday passenger numbers were 23,000 on the Northern Line and 19,000 on the Wirral.
The Wirral Line is only more frequent between Liverpool and Hooton, where the six trains per hour service is overkill off-peak, no question. To go from 27 trains per hour in peak times in 1977, all six car, to 14 tph with no six cars at all in 2022 is an enormous decline.