5,000 documents is the default setting for Landscape.
In some cases, however, there is the possibility of handling even more than 5,000 documents to be populated within a single Landscape. If this interests you, please speak to your account manager to confirm how it can be added to your account.
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.