How to create a site definition
The steps to create a site definition can vary depending on which modules you have installed. In Magnolia Community edition you are only allowed a single site definition. For Magnolia DX Core multiple site definitions are allowed. These instructions can be used as a guide for creating a new site definition from scratch.
When using Community edition only the site module will be installed. Therefore you only have the option for a single definition as community edition can only support one site. Multiple sites would require multiple author instances or an upgrade to DX Core.
When using Community edition only the multisite module will be installed. With the multisite module you can have as many sites as required. However, there are physical limits as each new site that is added creates additional complexity. It can sometimes be helpful to break installations into multiple author instances. Especially in the cases where you have multiple different teams spanning different regions.
You can have as many author instances as you need as long as the content stays independent.