I believe the original Agfa recommendation for Rodinal was 10ml per 135-36 roll (or about 80 square inches). A large number of users seem to do just fine at 5ml/80sqi. And some have whittled that down to 3ml/80sqi.
Keep in mind that those minimums must exist in a volume of water as a dilution ratio that works for your processing needs and equipment.
Worth noting as well is that at the current Freestyle price of US$13.99 for a 500ml bottle, using the Agfa recommendation results in a cost per roll for 135-36 of only 28 cents each. Some will try to economize by using less, but for the life of me I can't see why.