Can I assign one elastic IP to different subnets that are residing on different availability zones but same aws region?

You will need to have one Elastic IP per each subnet/availability zone. Please refer to the following documentation.

Note: Be sure to create the same number of Elastic IP addresses as the number of subnets that you select in each Availability Zone. For more information, see Elastic IP address limit.

• How do I attach an Elastic IP address to new or existing internet-facing Network Load Balancers?