What's shutter closure?

And how do I find the number of the shutter closure (speed) on my camera? I have a Nikon D5100 btw.

Shutter closure has nothing to do with shutter speed. Shutter closure determines
the amount of light allowed thru your shutter, Say a lens is maximum f2.It would
close down to 2.8 or 3.5 0r 4.7 or f6, etc. The smaller the shutter aperture, the
deeper the field of focus. You can use different combinations of shutter closures
and shutter speeds I have not seen a Nikon D500, so the info above is just basic.

The Martians have landed!

I think you may be just asking about how you find the shutter speed?

If so, the shutter speed usually always is a number on the display screen on Nikons. A number by itself is usually in a faction of a second. So the number 500 would actually mean 1/500th of a second from shutter opening to closing

When the display shows the " symbol, that means seconds - such as 3" means the shutter speed is 3 seconds.