I obviously didn't notice you had a Format-Table in there.
Just do
$ergs = Get-Pool | Select Pool_id,@{N="desktops";E={$_ | Get-DesktopVM | Measure-Object | Select -ExpandProperty Count}}
The Format-Table cmdlet is intended for displaying output.
As you can see from the returned types, these are special objects that are understood by the PowerShell output routine.
If you need to use objects and properties further along inside your script, do not format them (yet) for output.