How do I run a report that tells me how many invoices I have paid for a particular supplier? Thanks

It's nice to see you here in the community, @deb42. Let me guide you to pull up a report that will give clarity to your concern.
 

We can run a custom Transaction List by Vendor report to view the invoices you've paid for a particular supplier. You can follow the steps below:
 

1. Go to the left navigation bar and select Reports.
2. Locate and open the report Transaction List by Vendor. 
3. Filter Report period if needed.
4. On the upper right select Customize/Customise.
5. Click the Filter drop down arrow.
6. Go to Transaction type, beside transaction type click the drop down arrow and select Payment.
7. Then, Run report.
    

We can also pull up Bills and Applied Payments report, it will automatically show the bill and bill payment check transactions.
 

Here's how:

1. Go to the Reports tab.
2. Locate and open the Bills and Applied Payments report.
3. Filter the Report period if needed.
4. Select Run report.
    

To further assist about Running a report in QuickBooks Online you an check this article.
 

If you also want to customise a report you can check this article for more information.
 

Let me know if you need further assistance by posting a reply. It's a pleasure to assist you. Take care and have a good one!