That going to be a Development question and not ( currently) as CS question. Iād personally would suggest posting in Classic , as well as hitting up the developers on Twitter.
Unless, Iām confusing Classic with BC, I recall running two instances of Classic on separate wow accounts just for moving stuff cross faction via the neutral AH.
I believe there may be still a limitation for the current launcher where you can only have one instance of it active. It has an option to shut itself down after launching a game which enables you to restart it and launch another wow license. There was no launcher back in classic.
Looking at the launcher recently I can see you have an account list for classic now so I should be able to open Classic twice if I pay for two warcraft accounts on one Blizzard account.
As others have said, everything needs to be active. It doesnāt make sense that if have WoW1 active and WoW2-8 inactive, you get 8x the benefits of people with just WoW1 active. If you have WoW1-8 active, then you get 8 Classic accounts (1 for each active account) and each Classic account will have three slots available.
Also, Iām not 100% on this, but I believe the three character limit is simply for the reservation period starting on the 13th. Once Classic is live two weeks later, you will get more slots. I believe vanilla had 9 slots (Edit: per realm) at launch, so it would follow that 9 would be available. Again, not 100%, but itās likely this way to prevent people from name camping.