How to autopopulate a number for purchase orders?

Please elaborate in more details, some screenshots etc.

Are you talking about generating a unique reference number or rather increment a number + 1 each time a new form is submitted?

I personally, generate a reference number based on [CurrentDate][CurrentTimeSec]
for example: PCL04042019151605

List and Library entries already have a unique number assigned on creation ID. If its format is not what you need, then there are a few alternatives:

1. In the SharePoint site collection activate and configure the feature to create Unique Document IDS (see this blog)
2. Otherwise you can create your own list column UniqueID (or other name) and file it with a workflow that starts when a new item is created. Here you can have 2 flavours as well:
   1. Use the form's default ID for the sequential part of the number. However, if if you delete a form, you will end up with gaps in your numbering
   2. To resolve this, you need to keep track of the last/next sequential number in a helper list. In case you delete the last form, you need to decrease this number before the next form is submitted.

Most likely other community users have even more ways to implement this.