Multiple Nat Gateway in AWS VPC

AWS recommends to create multiple nat gateway in one single vpc.

AWS recommenedation message

To ensure high availability and minimize inter-AZ data transfer costs,
 we recommend utilizing separate NAT Gateways in each AZ and routing traffic locally within the same AZ.

Each NAT Gateway operates within a designated AZ and is built with redundancy in that zone only.
 As a result, if the NAT Gateway or AZ experiences failure, resources utilizing that NAT Gateway in other AZ(s) also get impacted.
 Additionally, routing traffic from one AZ to a NAT Gateway in a different AZ incurs additional inter-AZ data transfer charges. We recommend choosing a maintenance window for architecture changes in your Amazon 

Some good information provided in the belwo stackpver flow document.

Please go through that

Can a single NAT Gateway span across multiple AZ?

credit goes to : the person who gave answer in that document.