Three sources for pressurization are a plenty for the two packs installed.
In addition, for the times when
all turbocompressors fail enroute (an unlikely occurance), 9th stage bleed air is available on most models, expect the freon air conditioning equipped aircraft.
Have personally (when maximum range was required), switched OFF all turbocompressors and used engine bleeds for pressurization (3% improved specific fuel consumptioin). This is OK for freighters, but results in rather low cabin ventilation rate with pax aircraft.
IIRC, the 707 used by the USAF to transport the president, had 4 turbocompressors installed.
Nothing but the finest for the boss, I guess.
