Gets the total item count of all items in a SharePoint 2013+ content database.

Get-TotalItemsInContentDatabase.ps1

$sites = Get-SPSite -Limit All -ContentDatabase "SP2016_CONTENT_001"

$itemCounts = @()

foreach( $site in $sites )
{
    $itemCount = 0

    $lists = $site | Get-SPWeb -Limit All | SELECT -ExpandProperty Lists 

    $itemCounts += [PSCustomObject]@{
                        Site       = $site.Url
                        TotalItems = $lists | Measure-Object -Property ItemCount -Sum | SELECT -ExpandProperty Sum
                    }

    $site.Dispose()
}

$itemCounts

"`n`nTotal Items: $($itemCounts | Measure-Object -Property TotalItems -Sum | SELECT -ExpandProperty Sum)"