I remember when the sizes were 12, 20 and 36, then the 20 was replaced by 24, and the 24 on the frame counter was often in red. On much older cameras, it’s the 20 that will appear in red.

I feel your pain! My scanner does four strips of six, and it’s the same problem.

That is showing that frame 9 is ready to take (assuming you’ve just wound on the lever). It will show even numbers with a dot between them for the odd numbers, because there’s not enough space to show every number. If you press the shutter and advance the lever, it will also advance the number in the window to 10. This is how you know how many pictures you have taken on the film.

Look on the top of the camera next to the shutter button. There a little window with numbers in it. This advances by one each time you take a picture. When it reads “36” you will be on the last frame of the film (though in practice you might be able to squeeze in one more).