How do I find the last invoice number issued?

I'll share the steps to check the last invoice number in QuickBooks, @zapkvr.
 
Invoice numbers in QuickBooks Online are sequential. The next invoice number is based on the last and will automatically show when you create an invoice.
 
I'll give you options in checking the last invoice number.
 
The first is to click the search icon at the upper right of your QuickBooks Online account. This will show the recent transactions created or processed. See the sample screenshot below:

You can also see it when you run the Invoice List report. It will show all the invoices. You can sort the transaction number to show the latest to the oldest invoices.

Here's how:

1. From the left menu, go to Reports.
2. Enter Invoice List in the search box.
3. Click the No. column header to sort the transactions from the latest to the oldest.

Another way is from the Sales menu. Go to the invoices and click the Date column to sort the transactions.

When customers pay their invoices, you can start recording their payments. You can browse this article for instructions and detailed steps: Record invoice payments in QuickBooks Online.

Let me know if you have questions about managing invoices in QuickBooks Online. I'm always here to help. Have a great day.