When Annual maintenance Charges will be deducted? And how much?

I mean which month exactly? What is the amount?

I got this message from Zerodha.

This is to inform you that your account is due for Rs. 75 + GST towards quarterly demat account maintenance charge (AMC) and will be charged today. Kindly check your funds statement to ensure that you have sufficient balance. Any debit balance in your account may attract interest charges.

The AMC charge will be visible on the monthly invoice. You can download this invoice from the Reports section on Console.

I only have Rs 2 on my Zerodha account and it will take 3 to 6 days for me to add funds.

It is already mentioned in message to you that AMC will be charged today and charges are 75 + GST at 18% it will be 13.5 so total charge will be 88.5.

In this case you account will go into negative balance and you will be charged interest of 0.05% per day on negative balance until you add funds.

But AMC is 300? Also, is July 21 is the date of payment every year?

AMC is 300 + GST annually but is charged quarterly 75 + GST (75 * 4 = 300 + GST).

Date might be counted from the day you opened the account.