Found it, if anyone is interested.

Tools, Options, View, deleselt "Windows in Taskbar"



"StephanieH" wrote:

My entire system crashed a couple days ago. The IT guys reimaged the pc and
I've lost a lot of settings.

Now, Excel always opens a new session anytime I open a workbook. In other
words, each workbook appears separately on my task bar. I'd like to have a
single session of Excel with all the workbooks in it. How do I force it to
open the workbooks in the same session/instance?