As Dave stated, there is supposed to bewasher on the drain plug. It is what is called a "crush washer" and is designed to flatten out and conform the the sealing surface. That's why he suggested a dealer for the part.
As for quantity, if it's your GL12, I think my book recommends 3.7 U.S. quarts if you change the filter. And I do hope you are changing the filter.
Dump in three an half quarts, crank the engine with the kill switch off until the oil pressure light goes off and then add oil to bring the level up between the marks on the dipstick. Just be sure the oil you use doesn't say "energy conserving" or "friction reducing" on the API donut symbol on the bottle. I always use DELO 400 15W40 on my bikes. Good heavy base oil which is designed for diesels so it's more resistance to viscosity breakdown. I like to change oil at 4000-5000 mile intervals which is a bit more often than Honda's recommendation of 8000 miles.