You do not need to maintain any minimum balance to keep your Zerodha account active. Your trading account becomes inactive or dormant only if you do not trade for 24 consecutive months, while your demat account remains active indefinitely.
Zerodha charges an Account Maintenance Charge (AMC) of ₹300 + 18% GST per year on your demat account. This amount is deducted from your Zerodha account balance every quarter from when you opened your account.