AWS LightSail is different from shared hosting providers, where AWS provides Virtual Private Servers(VPS) with its own IP.
Generally VPS is for a single deployment of an application(e.g Single Wordpress Deployment per VPS). So it is recommended to get multiple VPS for your deployments unless your applications are really light weight and gets less traffic.
There is also a limit where you gets 2 VPS servers per AWS account by default. If you need more VPS, either you can send a support request to Amazon and increase the limit or use multiple AWS accounts (If you own the AWS accounts, you can setup consolidated billing)
In DNS setup, since its single application per VPS, you need to create a A record for each subdomain.