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.
To be frank, even if they tell us, what happened, we shouldn't trust them and should not be using that wallet again.
Even if they were not hacked, it is still not recommended to use closed source wallets like Atomic wallet or Trust wallet.