Hi there, @d-sylvain-coughl.

Thank you for posting in the Community. I'd be happy to help and walk you through on how to see the check number when reconciling. 

You can add the Reference number column to view the check numbers on the Reconcile window.

Here's how:

1. At the top menu, click the Gear icon. 
2. Select Reconcile.
3. From the Account drop-down menu, select the appropriate bank account.
4. Click Start reconciling or Resume reconciling.
5. Enter the information requested.
6. Click Start reconciling.
7. On the Reconcile page, click the small Gear icon. 
8. Under Edit columns, put a check in the Ref no. box.

I've attached an article and screenshot for additional reference: How to reconcile accounts.

That should take care of it. I believe that you'll see the check numbers on the reconcile page in just a few clicks.

Also, The phone support agents will be able to assist you in getting this resolved.

Here's how you can reach them: 

1. Go to https://help.quickbooks.intuit.com/en_US/contact.
2. At the top right, select your QuickBooks version.
3. Select a topic.
4. Click on the Get Phone Number or Start a Message button.

Drop me a comment below if you have any other questions about reconciling in QuickBooks Online. I'll be happy to help you further. Have a great day.