You can't merge placement groups. An instance can be launched in one placement group at a time; it cannot span multiple placement groups. On-Demand Capacity Reservation and zonal Reserved Instances provide a capacity reservation for EC2 instances in a specific Availability Zone.

How many instances does a placement group have?

You can have a maximum of seven running instances per Availability Zone per group. If you start or launch an instance in a spread placement group and there is insufficient unique hardware to fulfill the request, the request fails.

Can a placement group be deployed across multiple availability zones?

A spread placement group can span multiple Availability Zones, and you can have a maximum of seven running instances per Availability Zone per group. Spread placement groups are recommended for applications that have a small number of critical instances that should be kept separate from each other.