Active Directory count

 (Get-ADUser -Filter *).Count
(Get-ADComputer -Filter *).Count
(Get-ADGroup -Filter *).Count

$ADUser = (Get-ADUser -Filter *).Count
$ADGroup = (Get-ADGroup -Filter *).Count
$ADComputer = (Get-ADComputer -Filter *).Count
$ADObjects = $ADUser + $ADGroup + $ADComputer
$ADObjects

Count for specific OU:

(Get-ADUser -Filter * -SearchBase "OU=Users,DC=example,DC=com").Count