Many years ago I used to work for BBC Sound in the transfer and dubbing areas...
We had Albrecht 16/35mm mag film bays, Otari and Studer 24track 2" machines, Nagra T and 4S 1/4" machines plus a host of video stuff.
My personal record for sync was 19 Albrechts playing 16mm tracks onto the Otari, with 6 of those tracks also going to Nagra Ts, plus an Albrecht with film head playing pictures and 2 mixes from the 16mm mag tracks onto D3 and BetaSP video machines, and those pictures and main mix going onto a bank of 10 VHS machines.
Took me 1.5 hours to set up - serious plugging job - but was a fine sight with all those machines playing and recording in perfect sync!