Coupons...again May someone help me please?

Alright so another couponing question.... I am defiantly a beginner at this and still don't fully understand how people save $800 dollars on shopping trips. Okay so I'm going to try and explain this the best I can, so on a coupon when it says "one coupon valid for item(s)", "limit one coupon per purchase" or "coupon cannot be doubled" this means more then one coupon may not be used for one item? (ex: Sea Breeze, says "limit one coupon per purchase $ 1.00 off) so I can't use 5 or 6 coupons on the single bottle of Sea Breeze? And if all my coupons say it how do people save that much money? Also, a recent coupon expert I contacted said that on the bar code on a coupon, if it begins with a 5 it will automatically be doubled and if it begins with a 9 it will not. Is this true? and What does doubled mean? and Is the first number the actual beginning number of the bar code on the outside of it or does it begin within the bar code?