You can add multiple lanes to a pool simultaneously.
When you move or delete objects the size of pools and lanes adjusts automatically.
Lanes and pools can only be selected by clicking their border or name as soon as objects are placed in them. When you delete a lane, the size of the pool (or the wrapper lane) decreases automatically.
Procedure (quick video)
You have added multiple lanes.
Tip
To add multiple lanes, you can also click the icon for adding. It is displayed when you click in the pool header.