Step::1 Login to your WHMCS interface.
Step::2 Hover over the Setup section to Products/Services and click on products/services option.
Step::3 Now go to the Product and Click on the Edit icon.
Step::4 Onto the next page hover over Module setting tab and click on Switch to Advanced mode option.
Step::5 Now fill out the details and set your custom limits.
Step::6 After getting it done click on Save changes button.
The above given steps are in more detail in the video tutorial. You may check the blog as well.