A single main subject or a 'punctum' that defines the centre of your particular engagement with the photograph is the norm, and sometimes hardens to a rule, but it's not the only way, especially in as broad a topic as composition.

I have been inspired by abstract paintings and by the art of China, Korea, Japan and elsewhere in the Far East, all of which often lack a single main subject or centre of interest, and which can be filled from edge to edge with fine detail. For me, the distinction is between a photograph that shows a single thing or event, and one that shows a state of being or an environment.

For a canonical photographic example see Winogrand's "World Fair" bench.