So normally exchanges divide wallet to Hot and Cold. Right?
Then for small withdrawal request from users, exchange's Hot wallet automatically process it,
and for big amount of withdrawal request, exchange's admins manually screening it and send from Cold or other wallet.
If this is right, then normally how much BTC (and other Alt-coins) exchange divide between Hot and Cold, how is their ratio?
10%(Hot) / 90%(Cold)? or?
Yep. Hot wallets facilitate normal withdrawals and cold storage isn't normally touched unless the hot wallets needs replenishing, or there is a very big withdrawal that needs manual approval and hot wallet alone doesn't have enough funds.
According to
https://support.bitfinex.com/hc/en-us/articles/213892469-Bitfinex-Security-Features, Bitfinex claims that 0.5% of their coins are in hot wallets and around 99.5% are in cold storage. Of course, these figures can be inaccurate or exaggerated to make them seem more secure.
Coinbase claims that they store 98% of customer funds offline:
https://www.coinbase.com/security?locale=en. Again, nothing verifiable but it shouldn't be that far off from the accurate figures.
I would say that anywhere between 80-99.5% would be normal depending on the size of the business and the frequency of withdrawals, as well as their business model.
