How Do I Remove Those Numbers from Kubuntu's Task Manager?

Short answer: I afraid you can't remove only <2> or <3> from appearing. There has to be a way to distinguish windows.

There is a workaround which I like and that is to use an alternate task manager which is available by default on a normal Kubuntu 18.04 system.

First, I see what you see with the default task manager with three Google Chrome windows:

With the default task manager, there is a way to collapse the three separate entries into one. See the answer by cipricus for more detail:

I prefer the alternate task manager widget^* known as the Icons-only Task Manager:

^*Note: to access the available widgets, click on the ☰ icon usually located at the right-end or bottom of your panel.

I also prefer a vertical panel and the images below are with a vertical panel:

Icons-only Task Manager with one Google Chrome window

Icons-only Task Manager with three Google Chrome windows

Notice the little green circle with a white plus-sign next to the Google Chrome icon. That indicates the presence of more than one window open for a particular program. The three entries to the right of the Google Chrome icon are there because I single left-clicked on the icon. I can then select the window I want to maximize (or whatever). Otherwise, those entries are not visible.