There are a few different ways that you can determine how many windows are open in Excel. One way is to simply look at the number of tabs that are open at the top of the Excel window. If you have multiple workbooks open, each workbook will have its own tab. So, if you see five tabs, then you know that five windows are open. Another way to determine how many windows are open is to use the keyboard shortcut Ctrl+Shift+F6. This shortcut will cycle through all of the open windows. As you press the shortcut keys, you will see the active window change. The number of times that you press the shortcut keys corresponds to the number of open windows. For example, if you press the shortcut keys three times, then you know that three windows are open.
You can also use the Window menu to see a list of all open windows. To do this, click on the Window menu item and then hover your mouse over the Arrange All option. This will cause a submenu to appear with various options for arranging your windows. One of these options is Cascade. When you select this option, all of your open windows will be stacked on top of each other in an overlapping fashion. This makes it easy to see how many windows are open because you will be able to count them. Another option from the Arrange All submenu is Tile Horizontally. This option will arrange all of your open windows side by side so that you can see them all at once. Again, this makes it easy to count how many windows are open.
If you want a more detailed view of all of the open windows, you can use the Task Manager. To do this, press Ctrl+Alt+Delete on your keyboard and then click on the Task Manager option. This will bring up a dialog box with various information about your computer's performance. Click on the Processes tab and then scroll down until you find the excel.exe process. This process represents Excel and it will have a number next to it that indicates how many instances of Excel are running. Each instance corresponds to one open window. So, if there are four instances listed, then four windows are currently open.