You likely didn’t change any setting—the P900’s aperture is physically limited to f/8. On this camera’s small sensor, stopping down beyond that would increase diffraction and noticeably soften the image, so Nikon doesn’t provide smaller apertures like f/11.

So yes: higher f-numbers mean a smaller opening, but on the P900 f/8 is as narrow as it goes.

For fireworks, if you need less light or a longer exposure, use other controls instead:

• lower ISO to 100
• use a longer shutter speed as appropriate
• add a neutral density (ND) filter in front of the lens if needed

In short, f/8 is the camera’s limit, not a setting you accidentally changed.