You do not need to maintain a minimum balance in your Upstox account after you have purchased a SIP for the first time. 

For an ongoing SIP, each installment amount will be directly deducted from your bank account. Please note that as per the Exchange policy, the SIP amount will be deducted on a working day (this does not include the weekend and public holidays) prior to the due date of the SIP. Kindly maintain the required balance in your bank account to avoid any future penalty charges, that your bank could charge. Also do check the return instruction charges with your bank to stay informed.