By default, TomEE will grab 2GB of memory. This can be adjusted in /bin/setenv.sh. The default value should be fine to get started, you might want to increase this to 4GB if your repository grows a bit, but rarely beyond that.
That would leave 4-6GB for the executor, which, to be honest, is not recommendable at all. Depending on the number of users working on the server in parallel, or parallel execution of workflows, having that little memory at your disposal will be a severe bottleneck.
Based on 5 users, I would not recommend to to use an instance smaller then 32GB, better go for 64GB. Our official recommendation is to use an r5.x2large instance.