dont ask why
Well it is impossible to give the best answer without knowing the reason why you want to create this many "wallets" per day!
For example if it is some sort of project to "get lucky" and find a funded address then it is a waste of time but also the answer to such question is that you shouldn't create "wallets" but to create keys and in such a scenario you won't need to store them so there is no need to discuss databases, etc.
But if the project is for some sort of centralized service then things are very different, you do need to create "wallets" and you do need "storage". But then again the details depend on the type of the service. What you would do for a custodian web wallet is different from non-custodian web wallet and different from an exchange and also different from if you want personal wallet for a service receiving payment (last one needs a single wallet with many "child" keys).
In these cases you would need to discuss database (what to use and how).
I can keep going but I think you get the gist. The other replies here are also all over the place because they don't know your purpose.
P.S. Although with "10 mil/day" I'm guessing your goal is the first thing I said!
You can try with smaller number, repeat it a few times. I try with 100000 and it works.
Electrum wallet is not designed to handle large number of keys/transactions so I seriously doubt it can perform well for anything beyond a couple of hundred keys.