Hi Indrani Sharma,
You need not create any new flow for this. just a little bit of customization is enough.
The user role selection in the Account_New page is a group box widget. Add an On Change event, retrieve the user role entity with the association, find the count (aggregate list activity), remove the latest user role object from the association, and show an info message.
I would suggest duplicating the account new page and placing it in your module and adding your customization there since updating the module will lose your customizations.