I am not sure what the amp value is but is it higher than you would
imagine.  I would guess 7-20 amps. You excite the alternator field (couple
of amps) heat the 02 sensor, run the fuel pump momentarily/intermittantly,
run the computer, ignition components etc. In Westies it is a good idea to
power the radio off something other than Ignition + power.  I wire it hot,
but I have seen other people put switches to switch between Ign + and hot
battery.
    VW uses D+ on the alternator (runs the charge indicator light) to also
power a relay or 2 to keep you from running your battery down  with the
ignition on.  The little relay under the driver seat (Westy and Multivan
only) powers the DC fridge mode only when the engine is running.

   John