The more characters you have, the more gold you can store in the bank.
The maximum storage capacity for your bank is 20,000,000 Gold per character that is created on the account. This can be increased by 20,000,000 for each character you have on your account, for up to 10 characters.
After the 11th character, the bank limit is increased by 10,000,000 gold per character created on the account.
You will not be able to deposit any more gold to the bank once the maximum gold is reached for your account.