Do we have the data revealing how many users were using the atomic wallet before the hack and how many people are still using this wallet?
No, we cannot get that data, but Atomic wallet claims that the hack only affected less than 0.1% of their userbase, so if that is true, maybe so many people who don't know the right thing to do would still be using Atomic wallet.
Everyone who hears this news should stop using it even if they do not fall in this 0.1% category. Once hacked, it can be exploit again either the the hackers or by the team themselves.
Even if they were not hacked, it is still not recommended to use closed source wallets like Atomic wallet or Trust wallet.
Trust Wallet is even more widely used than the Atomic wallet. By the way, if all closed source wallets are not recommended, why do we use/promote Ledger Nano (Hardware Wallet) ? It is also a closed source as I know.