Correct, there are 53 777s (7 of which battery), and (based on a quick look at the Saturday timetable) the count per line is as follows:
Headbolt Lane - 4 (requires battery)
Ormskirk/Hunts Cross - 8
Southport - 8 (ignoring the exiting doubles)
Chester - 6
Ellesmere Port - 3
New Brighton - 4
West Kirby - 5
So a total of 38, meaning you would need at least 76 units (at least 8 of which with batteries) to run as all doubles, and that doesn't account for spares, long term out of service, exams, further expansion, etc.
Last Saturday (yesterday as I'm writing this), there were 4 doubles on Southport services, to 42 units in service (79% of the fleet)