For the most part, people don't buy 2 sets of 36 diapers.

Honestly, some people like sized diapers, I like One Size. Except for my prefolds and most of my covers, the rest of my stash is used.
I like to go a week between washings, so I have quite a bit of 'fluff'.
Are your babies usually bigger or smaller than 'average'?
If it were me, knowing what I know, I would get myself a bunch of prefolds and newborn sized covers (my babies are small). I would also get a nice big amount of Bumgenius One size pocket diapers. They are smaller than the Happy Heiney One size diaper, which would be fine for my babies. If your babies are larger, then skip Bumgenius, and head towards Happy Heineys One size.
I wouldn't really get any other sized covers unless you 1) like prefolds, and 2) your baby is growing so you can tell if he has super chunk thighs that need an accommodating cover (like Prowraps) or super skinny thighs that need an adjustable cover (like Motherease Air Flow).
Ds, for example, was just shy of 5lbs at birth & short. He super chunked up though, and at 6/7mos he had a 22 inch waist!!! By that age, he was in larges. I even bough XL thinking he was going to outgrow his larges.

He was completely unsnapped on both the Bumgenius and the Happy Heineys. However, he started slimming down starting at 11mos, and is now tall and skinny again. He also has really skinny thighs (11 inches), so side snapping covers are a better choice (can get smaller thigh openings that way, with less droop than wrap covers, aka the ones that wrap to the front).
Due to the wrap style front of both the Happy Heiney's and the Bumgenius, I can't really get the thigh tight to prevent leaks. So right now, I have some Bummis pull ons and some side snapping covers coming to me, and am happily using my prefolds.
If your babies grow much more normally than mine, then the one size covers should totally be fine. To talk in sposie terms, ds was in preemies until 1 mos, then in size 5 at 7 mos, down to a size 4 by 12 mos and now, at 28mos he's downgraded into a size 3 due to skinny thighs. Yep, he was leaking out of his sposies.

I think Bumgenius is having a clearance sale on their BG 3.0s (newest version) right now, just in case. Also, the best price I've been able to get on diapers is at Costco. And they increased it so it's more like $40 a month. And I only use sposies on outings (like 1-2 times a week) and overnights. So, at most, 3 diapers. A box lasts about 1.5 months here.
Ami