I have created a content type and added an image field with unlimited option. Now I want to setup a views slideshow. Some nodes have single whereas others have multiple images. So my slideshow should show single image as well as all the images if the node has multiple images grouped together.
Try field slideshow. It bypasses views entirely, which makes it easy when all your images are in one node.
I don't think it works with the media module, yet, and you have to set your field type has to be "image". You select the slideshow options in the display fields portion.
jags, if you are using the Media module, this might be what your looking for: http://drupalmedia.freeworldmedia.nl/book/howtos-guides/how-use-views-slideshow-and-media-2x-module-create-slideshow
True, it's actually for the 2.x version that's still in development, but it offers a lot of information that's also still kinda valid for Media 1.x.
You can group images together in Views, edit the Image field in Views, you'll see an option for 'multiple values'. That's where you can define this.
In your view, navigate to the image field > Multiple field settings and check Display all values in the same row. Then for those nodes that has single image, it'll show one. For those nodes that have multiple images, it'll show all of them.