powershell numbering excel fields automatically

Good day folks I have a challenge that I have been given and need some assistance in resolving this issue. I have an excel spreadsheet with the following columns named emailaddress and batchnumbers. What I would like to do is to be able to on a fly be prompted for a batch number. The batch number depending on what it is will uniquely distinguish those group those users to the same batch. For example lets say I want to batch/group the users below every 5 users.

emailaddress                            batchnumber
user1@microsoft.com             uniquebatch1
user2@microsoft.com             uniquebatch1
user3@microsoft.com              uniquebatch1
user4@microsoft.com             uniquebatch1
user5@microsoft.com             uniquebatch1
user12@microsoft.com           uniquebatch2
user31@microsoft.com           uniquebatch2
user14@microsoft.com           uniquebatch2
user18@microsoft.com          uniquebatch2
user23@microsoft.com          uniquebatch2
user33@microsoft.com          uniquebatch3
user40@microsoft.com          uniquebatch3
user19@microsoft.com          uniquebatch3
user2@microsoft.com            uniquebatch3
user31@microsoft.com          uniquebatch3


Or if I wanted to batch 50 users per batch it will automatically be prompted for the amount of users and automatically do this. Any assistance would be great, thanks