It really depends on your system and file formats you choose to be honest. Poser itself doesn't have a limit to allowed number of polys per an object as far as I'm aware. However, certain file formats do - 3DS, the older version of LWO, etc. Additionally, if you're rocking an older system, your OS resource usage may limit the available memory to Poser and thus, limit the amount of polys you're going to be able to fit in a scene. It's practically a case-by-case scenario, so there's no hard and fast limit. There was a car, as I recall, that was once part of Poser that was over 100,000 polygons or so and back in the day would slow everyone's systems down. Now, my system laughs at 100,000 polys. So it varies.

 

A good content creator can keep poly count low initially but then there's the problem - at least with characters - of what different things folks may want to do with the figure. So you end up adding polys (since Poser doesn't have SDS - how I long for Catmull-Clark). You have to find a nice balance between a mesh that lends itself to morphing while at the same time stays light on system resources. It's tricky but as computers get more powerful and new (well, old really) technology gets added like sub-division surfaces, it becomes less of an issue.