In your case, you could potentially use a “seed” (for lack of a better technical name) record in the database.
Specifically, you create a table which will only store one record. This record will be used for holding a very specific value or two. (Ie. a number-based field which stores the total count of invoices created).
As part of your workflow, you can “Do a search” for the seed record’s invoice count. Then, you can have a workflow that increments the search count by 1 for each invoice created. Then, the create a new thing (Invoice) would reference that item’s invoice count as you assign a number.
From Bubble’s operations standpoint, you should be OK to do this without fear of the data being updated properly (see this post).