This question falls under marine engineering.

composed 2or3 person to monitor and maintain the all machinery to avoid accident

The number of people needed to take deck watchkeeping depends on the size of the ship. Watchkeeping means to operate a ship continuously, so technically you would only need two people to work 12 hour shifts.

You can check in dg site press icon - indos/cop and give the information of indos number and date of birth

Nine months for coastal vessel and six months for foreign going vessel