If your loading them in as Poser formats then you can run out of RAM pretty quickly, however you can save yourself quite a bit of RAM if you use DS formats.

To get DS files, load the figure into an empty scene (no camera or lights), apply the texture and morphs you want, then load in the hair and clothing, once your happy with how it looks save it as a scene file (.daz), "file>save as>scene.."

Once saved clear the scene, now you can load/merge the .daz file you just created as often as you need, you'll still run out of RAM if you go crazy, but you'll get more figures in the scene before you do, this is because in DS native format the program only loads the morphs that are non-zero, so all of those morphs in the hair, clothing and figure you didn't use get left in the 'data' folder.