If your salary is Rs 20000, obviously, you will get Rs 20000 every month. If this 20000 includes any reimbursement, like travel or mobile, then that amount will not be available as you will not be traveling or using phone during maternity leave days.
If you are covered under the ESI schemes, the payment will be made by the ESI Corporation and the amount will be 100% o the amount which qualifies to ESI (the amount on which your company deducts ESI)