In VBScript, how can I retrieve the first element of an InstancesOf collection?

The GetObject("winmgmts:") returns a SWbemServices object. According to the documentation for the SWbemServices object the InstanceOf() method:

^{From SWbemServices.InstancesOf method}
creates an enumerator that returns the instances of a specified class according to the user-specified selection criteria.

The idea of an enumerator is to enumerate over a collection of objects this lends itself to the VBScript For Each statement for iterating over an enumerator.

A simple example would be;

Dim swbemInstances, swbemInstance
Set swbemInstances = GetObject("winmgmts:").InstancesOf("Win32_OperatingSystem")
For Each swbemInstance In swbemInstances
  WScript.Echo swbemInstance.Caption
Next

You can access an instance directly from the enumerator using the ItemIndex method which as the documentation says;

^{From SWbemObjectSet.ItemIndex method}
returns the SWbemObject associated with the specified index into the collection. The index indicates the position of the element within the collection. Collection numbering starts at zero.

Note: Interesting point the documentation actually cites the Win32_OperatingSystem class as an example where you likely want to retrieve only one instance and explains how to use ItemIndex to facilitate it.

^{From SWbemObjectSet.ItemIndex method - Examples}
Only one instance of Win32_OperatingSystem exists for each operating system installation. Creating the GetObject path to obtain the single instance is awkward so scripts normally enumerate Win32_OperatingSystem even though only one instance is available. The following VBScript code example shows how to use the ItemIndex method to get to the one Win32_OperatingSystem without using a For Each loop.

Something like;

Dim swbemInstance
Set swbemInstance = GetObject("winmgmts:").InstancesOf("Win32_OperatingSystem").ItemIndex(0)
WScript.Echo swbemInstance.Caption

^{Also mentioned in the comments}