The system can assign all new members a membership number.
You can define a prefix for that membership number, either for the organisation as a whole or a different pre-fix for each Membership Category.
You can set which number you want the membership number to start from and once membership numbers have been allocated, the system will show the next number to be issued.
Be sure to include the Membership number field when building your forms.
When new members register and pay by GoCardless, the membership number will only be published for the member once the funds have cleared (ie. moved from 'pending' to 'paid').