short answer? maybe as many as 20 car alternators, but
time you buy 20 from the junkyard, buy the special pulleys needed, and the belt
to drive them all (cog drive and double cog serpentine capable belt) and then figure
a way to regulate and control 20 alternators worth of field current (about 60-80amps)
he will end up with the following
and incredibly rube goldberg, horribly inefficient, and more expensive than it would be
to just go out and buy a 30-40kwatt ST head from Georgia generator
i am sure if he has to ask he has no clue as to the drive requirements, the mounting requirements,
or the insanely expensive field control regulator he will need to control this frankencharger.
and don't even think about using a reostat to control the field, it alone would be massive and bloody expensive
and there is no way he can control the current to the point of stable power generation by hand
it just isn't going to happen. a custom built regulator to control 20 fields would likely cost several times what a 30kwatt
st head could be bought for.
twenty alternators that are for instance 100amp units, equates to 2000amps, that alone will require massive cables
which cannot be connected directly to any of the alternators directly, so a buss bar and feeder cables would have to be
setup.
and i haven't asked yet what voltage is he wanting to generate, 12volts?
don't get involved with this project, nothing good will come from it.
ymmv
bob g