If your salary is Rs 20,000, obviously, you will get Rs 20,000 every month. If this Rs 20,000 includes any reimbursement, like travel or mobile, then that amount will not be available as you will not be traveling or using the phone during maternity leave days.
If you are covered under the ESI scheme, the payment will be made by the ESI Corporation, and the amount will be 100% of the amount that qualifies for ESI (the amount on which your company deducts ESI).