You can simply drag your clone to the desired location while using the scroll-wheel to determine the amount of clones you like. The process is finalized by pressing the Enter key.
The preview models share the same instance of the source model, rendered with an offset for each clone. The translation is snapped to the current grid size, alternatively you can use precision-mode (Shift) to fine-tune your clone-offset. Works with any type of placeable entities, however entities without a model (particles etc.) use simple boundingboxes/icons to show their destination. This may change in the future, where even particles may be fully rendered while previewing.