Is it required to create a NAT instance in every availability zone of a VPC

I'm trying to figure out what are the best practices to setup a HA VPC

The question relate to:

1. 
   

   What is the correct setup of a VPC that is spread all over the AZ of a region, with private and public subnet in every AZ, is it mandatory to create a NAT instance in every public subnet ?

   
   


2. 
   

   How to achieve HA for the NAT instance, i.e should i wrap it with a autoscale group that ensure that i will have 1 instance up and running ?

   
   

Thanks!