You can select one or more models and copy/paste them using the buttons in the top toolbar or with the familiar shortcuts:
Ctrl + C : Copy
Ctrl + V : Paste
Object copies include everything from modifiers to object instances or support blockers and enforcers.
You can add or remove instances by selecting a model and then clicking on the plus or minus icons in the top toolbar or using the keyboard shortcut:
+ : Add instance
- : Remove instance
This option is also available in the context menu when you right-click on a model. The context menu also has the option Set number of instances, which is useful when you need to print many smaller objects.
Copy Ctrl+C and Paste Ctrl+V do not create instances but copies independent of the parent object.
Fill bed with instances feature fills the print bed with the maximum number of copies of a selected object. This feature is accessible from the context menu after right-clicking on an object.