I was under the impression that modern stamps were printed with the same
plate used thousands of times, but the "plate number" changing
automatically
as each sheet is fed thru or being added later - much like the serial # is
put on paper money. Also what about the printing methods that do not use
plates? SO it's not really a plate number but a sheet number. Or is my
idea
wrong?

The number that appears on a plate block is the plate number, not a serial
number of the sheet the block of stamps comes from. Plates used in printing
bills, bonds, stamps, etc., are numbered for control and audit purposes.
Stamp collectors are interested in the plate number because stamps printed
from different plates, even if they have the same design, have perceptable
differences.

The inscriptions on stamps that are not printed from numbered plates usually
contain information about the stamp's designer, printer, colours used (the
"traffic lights"), perhaps some explanatory info about the subject matter,
and, if more than one printing, as is often the case with definitives, a
print run number (i.e., a simple number like "1" or "2" indicating first
printing, second printing, etc.).