A single username can handle more than one account. As mentioned in the Connectivity section, the TWS will automatically send a list of managed accounts once the connection is established. The list can also be fetched via the IBApi.EClient.reqManagedAccts method. For an individual account, this call works as well and returns a single account.
Usage
reqManagedAccts(twsconn)
Value
Individual account: a string containing a single account number. For a FamilyAccount it returns a string with a ',' separated list of available accounts.