If you mean how many possible prefabs you can fit into one level, then you can fill a whole level full of them, I think. The engine dies because of dynamic physics, not really segments.
If you mean how many segments can you jam into a level, it's a 40x40 grid, about 15 stories. Do the math.
If you mean how many segments can FPSC take, as many as you can cram in, I guess. Dynamic stuff and high poly stuff usually makes the engine struggle.
If you mean how many prefabs
should you use, as little as possible. Use custom segment work, it looks much better then pre-set square rooms (the science lab is an exception, but predictable)
Though the segments should be basic, no complex poly counts or maps (diffuse maps may work, not sure 'bout normal maps, definitely no complex shaders).
My bad answer,
==MeZo
"Any gift can be great, no matter how small, because it's not the gift that counts, it's the meaning behind it"
"Heaven for climate, Hell for company"