im using the block upload image and i uploaded image and add a text in Captions but it's not showing  in the image any idea ?

If you use the browser developer tools to inspect that element, is the text possibly there but maybe behind the image or something along those lines? You may need to submit a ticket so we can take a look.

if single image will not show but if i upload more images  it will show ?

If you're finding that to be the case it's probably a bug. Click the bug tracker link at the top of the forum and report it.