From memory, the original design (a link to another Q site can be provided if you don't have it) had you drill 8 - 1/2" holes in a circular pattern into your lid. As an alternative, a lot of folks (including me) put a piece of pipe into their 2" bung hole for the open top barrel and removable lids they use. One pipe.
That is enough exhaust. Heat is regulated by the vents in the bottom. Too much exhaust space and the thing can start drawing from the top and run hot and stay hot. My lid has 2 of the 2" holes and if I leave them both open, one will draw, the other vent and I can get a hot fire going with all the vents on the bottom fully closed.
You can put in two racks. Without a dome lid, I would not put the top rack much closer than 8 inches from the top of the drum. Second lid 8 inches or so below that if you ever want to do mass quantities of pork butts, etc. For ribs only, enough distance for clearance. Remember with two racks, you will have to dismantle the stacks of racks to foil, etc, and all that takes time. Time with the lid open, during which the fire below is going to take off from the breath of fresh air.
If you go with more than one rack, do consider putting a baffle system above the charcoal basket to direct the heat away from the center of the drum. You will get a hot spot above the charcoal. A removable lid off a 30 gallon drum works for this or a plate of flat steel. Also consider a drip pan on top of the baffle to keep all that grease out of the fire.
Lastly, a lot of us put a pan under the charcoal. Catches ash, but also grease that falls and creates a mess in the bottom of the drum. Again, a removable lid off a 30 gallon drum works well for this.
The original design had the grease drip onto the charcoal to be burned. You can do that with only one butt, brisket, rack of ribs, etc. But fill a drum up with full and multiple racks and a lot of grease will get by and head to the bottom.
