I am using the Monero wallet RPC method get_balance to get my wallet balance.
One of the inputs is
account_index, which is described as:
account_index - unsigned int; Return balance for this account.
account_index and how do I get it?
Every wallet has a main account at index 0. If you created other accounts in the wallet, they also have an index, as shown in the wallet. Using get_balance, you can even omit the field
account_index as it will default to the main account (index 0).