How much store credit do my customers hold in total?

Go to Analytics → Reports and open Outstanding store credit balance (in the Finances category, or search for it in Search reports…).

It shows the current total of all customer wallet balances as Outstanding balance, split by Currency by default. You can add Wallet status to separate active and frozen wallets, and Accounts to see how many customers hold a balance.

This is a snapshot of today's balances, not a history. For credits and debits over time, open Store credit transactions in the same category.