It can be done in two ways:
1. Create a single Beanstalk application.
a) create different environments for each company inside the created application, and in each environment deploy the .war file and supply the resources as per your requirement like tomcat 8 or 9, the minimum number of instances & a maximum number of instances for Autoscaling. Health check monitor interval, no. of times to check before timeout etc.. and finally assign the cname (i.e, the URL by which you access the application) associated with the company name, like if the webapp is for abc company then provide cname as abc.elasticbeanstalk.com.
b) produce multiple beanstalk applications i.e., one for every company and in each application, you can create multiple environments like Development, Beta, Staging and live - based on your requirement.
2. And coming to DB
Go for RDS if your DB is a relational database. Two ways to plan for multiple companies is:
a) Create a single RDS and create multiple schema's in it i.e., one schema for one organization.
b) create separate RDS for each organization - recommended if DB records are more