Thanks for reaching out to us here. QuickBooks Online has a substantial list of reports, that you can customize, to see what matters most. I would be happy to guide you to the best fit, for viewing your monthly sales!
Follow these steps to view the Sales by Customer Summary report;
1. Open Reports from the left menu
2. In the Search field, enter Sales by Customer Summary
3. Choose the Report period
4. Use the ▼ dropdown to Display columns byMonths
5. Hit Run report
You're all set!
If you have any other questions, feel free to reach out again. We would be glad to help!