Hello,
If there are not enough funds, then the user withdrawal will fail and the user will receive an error message (that you can configure under “Wallets” -> “Notifications” -> “Notification settings for FAILED withdrawals”
You should probably hold more funds in the hot wallet than the largest balance on your site, because any user might decide to withdraw all of their funds at any time. The Cold Storage feature will tell you how much of each coin you should have online, so that you have a given percentage of the total user balances online.
If you have selected the option “Wallets” -> “Notifications” -> “Notification settings for ALL events” -> “Forward errors to admins” then you will know if a user has not been able to withdraw their funds.
with regards