Export-ContentDatabaseMappingsToCsv.ps1

Add-PSSnapin -Name Microsoft.SharePoint.PowerShell

$timestamp = Get-Date -Format FileDateTime

# Using Get-SPWebApplication here b/c Get-SPContentDatabase does not return databases that are not "online"
Get-SPWebApplication | SELECT -ExpandProperty ContentDatabases | SELECT @{ Name="WebApplicationUrl"; Expression={ $_.WebApplication.Url }}, Name, Server, WarningSiteCount, MaximumSiteCount, Status | Export-Csv "E:\_temp\DatabaseMappings_$timestamp.csv" -NoTypeInformation

Mount-ContentDatabasesFromCsv.ps1

Add-PSSnapin -Name Microsoft.SharePoint.PowerShell

$rows = Import-Csv -Path "E:\_temp\DatabaseMappings_20210624T1145590703.csv"

foreach( $row in $rows )
{
    if( Get-SPContentDatabase -Identity $row.Name -ErrorAction SilentlyContinue )
    {
        Write-Warning -Message "A content database with the name '$($row.Name)' is already mounted to this farm."
        continue
    }

    Mount-SPContentDatabase `
        -WebApplication   $row.WebApplicationUrl `
        -Name             $row.Name `
        -DatabaseServer   $row.Server `
        -WarningSiteCount $row.WarningSiteCount `
        -MaxSiteCount     $row.MaximumSiteCount
}