Code structure
Sites are based on HTML, mobile applications on code. If the underlying HTML templates or codebases are the same for multiple sites and applications, you may want to consider using a single tag property to manage multiple sites or apps.
Data
For all of your websites or applications, is the data you are going to collect very similar, somewhat similar, or unique?
If the data you need to collect is similar, it might make sense to group those sites or applications into one property to avoid duplicating rules or copying rules from one property to another.
If your data collection needs are unique for each site or application, it might make sense to separate them into their own properties. This method lets you control the data collection more specifically, without using large amounts of conditional logic in custom scripts.
Variables
Similar to data, are the variables you are going to set in your Analytics and other extensions very similar, somewhat similar, or unique?
For example, if eVar27 is used for the same source value across all of your websites or applications, it might make sense to group those sites or applications together so you can set those common variables in just one property.
Extensions, tags, and systems
Are the extensions, tags, and systems you are going to deploy very similar, somewhat similar, or unique?
If the extensions, tags, and systems you are going to deploy are very similar across your sites or applications, you might want to include them in the same property.
If you are deploying Adobe Analytics on only one site or application, and your other extensions and tags are also unique, you might want to create separate properties so that you have more control.
For example, If you are deploying Adobe Analytics, Target, and the same 3rd-party extensions across all of your sites or applications, that is a reason to group together.