5,000 documents is the default setting for Landscape.
The subset of patents that are landscaped are the top 5,000 search results based on whether you are looking at latest publication date (i.e. the top 5,000 most recently published patents), most relevant (i.e. the top 5,000 most relevant patents), and so on. As well as this, it will use the same settings as was used from wherever you created the landscape from.