How can I pick the last item of Property in powershell?

Here is my code:

Add-Type -AssemblyName PresentationCore
[Windows.Media.Fonts]::SystemFontFamilies | Select-Object -Property Source,FamilyNames

Actually, the property FamilyNames has many items and I only want to pick the last one.

How can I achieve this?

You do not want to list the last SystemFontFamilies, like you stated in your comment. You seem to want to select the property Source and the last entry in FamilyNames. If you examine what's contained within FamilyNames, you'll see it's a dictionary object.

[Windows.Media.Fonts]::SystemFontFamilies | Select-Object -expandProperty FamilyNames | Get-Member

TypeName: System.Windows.Media.LanguageSpecificStringDictionary

A dictionary will contain keys and values. If you inspect the keys/values, you should see something like this.

[Windows.Media.Fonts]::SystemFontFamilies | Select-Object -expandProperty FamilyNames

Name                           Value
----                           -----
en-us                          Cascadia Code
en-us                          Cascadia Mono

It seems you want to select the last value in Keys so I would use a calculated property to pull the desired info

[Windows.Media.Fonts]::SystemFontFamilies |
    Select-Object -Property Source,
                            @{n='FamilyNames';e={$_.FamilyNames.Keys | Select-Object -Last 1}}