Focus on the target, ignore the barrels.
One or two, depending on how many barrels the shotgun has.
both barrels are not choked
AFAIK, no.
Store brand shotgun with two barrels, side by side.
Try e-gunparts.com
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
No
Online
They probably denote what choke the barrel has.
Open the barrels and look on the flat part of the watertable (the flat part of the frame where the barrels close up) or the bottom of the barrels for the serial number
None