Print out all list names and the item count.

$site = Get-SPSite -Identity <https://mysharepointsite/sites/test>
foreach ($web in $site.AllWebs)
{
    foreach ($list in $web.Lists)
    {
        # Prints list title and item count
        Write-Output "$($list.Title), Items: $($list.ItemCount)"
    }
}
$site.Dispose()