[SOLVED] Unique ID-field for invoice PDF generation

Hi Drupalers, 

I´ve got a new question regarding my new website:
I have the content type “sold goods”. For this content type, I need a special field which automatically gets counted up by one called bill-id

For example, at the beginning, this number should be 1000 when the first node of this content type is created. If the next node is created this number should be automatically 1001, etc.

Unfortunately, I cannot use the node-id, because there are several users who use the system and who create content

Hope you guys have a good solution. I already looked for modules with primary key or something, but I couldn´t find anything useful 

Thank you so much guys! 


Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed