I've never adjusted the length of time based on number of eggs. As long as the water is boiling I don't think it would take any longer to cook a dozen than to cook one (it might take the water longer to come up to boiling, I guess).
For hard boiled I normally bring them up to the boil and then turn the heat off, and leave them for 15 minutes. If you like them less well-cooked, you could reduce this to 12 minutes. Then empty the pan and re-fill it cold water a couple of times to cool them down quickly. Leave rest of them sitting in the cold water while you prepare each one.
I don't boil eggs enough to keep a feel for the timing... So a few years back, I picked up a handful of these.
They're sold under a few different names, but the idea is the same: sturdy plastic that changes color as it heats. Drop it in with the eggs, and pull & chill them all when the color band hits the spot you're looking for.
My mother taught me this way: eggs not straight from the fridge but don’t have to be room temperature. Use a small saucepan with a lid. (However many eggs, use a saucepan big enough so the eggs don’t touch each other or the side of the pan.) Cover the eggs with fresh water & heat to boiling. When they get to a rolling boil, turn off the burner & cover the pan. Let them sit on the burner for 20 minutes. I get perfect hard-boiled eggs with this method with either electric or gas range.
Running cold water over them in the pan both makes them easier to peel and stops the cooking.
If you've got a sous vide setup you can set it for about 166ºF-ish for an hour and ensure they're cooked to perfection. However, you need to make sure that you've got it exactly that temperature as if you're too low the egg will come out runny. It's still safe, and arguably tastes better, but it's not hard boiled. In fact, this method doesn't boil the eggs at all, so I guess they're "hard cooked".
I steam them in my veggie basket, which holds 6-7 eggs. I read Serious Eats blog and learned this there. Uses same pot and stove as boiled, but not boiled. They recommend 12 min. Eggs are straight from fridge. I can stand the eggs so the yolk is centered for deviled eggs. They peel perdectly, which is why I now use this method. https://www.seriouseats.com/steamed-hard-boiled-eggs-recipe
I bought eggs identical in size from various breeds and there is no such a thing as right time to boil egg. You have to know how eggs, that you buy behave and be aware, that egg comes from raw to perfect in 30 second and from perfect to dust in next 30 seconds, so you have like 10 seconds to put all eggs in the pot at once and pay attention to removing them in same time every time you boil them. Lastly icebath. You don't want them to be ice cold, but you don't want them to be cooking any further.
I gave you more questions than answers, this is why the best boiled egg is poached egg. You don't peel, you see how cooked it is, it takes ~4 minutes to boil.