are double owners predominately hunters or collectors

For me they are a hunting tool. Can't afford to be a collector.

....do they own less firearms than the typical bolt-action rifleman?

Don't know. In Australia, probably more firearms because they are enthusiasts. In the USA probably not as every poster on these forums says something like "I've owned 7 .375s and 5 .458 Lotts ......"

How does a guy carry all those rifles at once when hunting?

...do they even own any turnbolts?

Yes. My guess is the average double shooter starts off with bolt actions.

...in what calibres?

Me. One only in .450 No2 NE but working hard to add a couple more - one bigger, one smaller

. For now

.

...do they also shoot double smoothbores?...

Yes absolutely. A side by side shotgun is a natural way to progress to double rifles. Maybe its all these U/O shooters that do to lever actions.