That limit is 6 or 8 light sources depending on the actual OpenGL hardware. The production renders do not have that limit. FireFly and SuperFly support as many light as you want. Well until you run out of memory any way.

You can pick which lights do or don't show in the OGL preview. There's an option on each light's properties to [  ] Include in OpenGL Preview. Just uncheck the lights you don't want included. When there are too many lights to display Poser will stop displaying the light's that haven't been moved for the longest time (First in, First out)