As a rule of thumb, I estimate a dressed up scene as .5 - 0.7 Gb, plus each dressed up character as 0.5 - 0.7 Gb as well. 

The 32-bit Poser is able to address max 3gb user memory, physical or virtual, when the operating systems allows it passing the 2Gb boundary. In Windows this requires setting the 3gb switch, to my understanding it's enabled in recent Mac OS's (and Linux) automatically. For Windows, I published an in-depth tutorial BTW.

3Gb gives room for one scene plus 3 characters (4x 0.7), or at most 5 characters (6x 0.5 Gb). 

My recommendation though is to render like the pro's: all characters separate, separate render passes for lights and masks as well, and the final compo in Photoshop. Don't do it in 3D if it can be done in 2D.

Happy Rendering

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though