We always for some reason need to find the count of the mailbox created in last week/Month etc. Below is the powershell command to get the count

$Result = Get-Mailbox -resultsize unlimited | where {$_.WhenCreated -gt (get-date).AddDays(-30)} | Select displayName
$Result.Count

To get the details to the text file you can use the below mentioned command to export to the text file

Get-Mailbox -resultsize unlimited | where {$_.WhenCreated -gt (get-date).AddDays(-30)} | Select displayName, Servername,Database, WhenCreated >> Result.txt