The number of system risers (that I know of) is unlimited; BUT - you must conform to "System Protection Area Limitatations" (NFPA 13, 2002 8.2) 52,00 sq ft (light hazard)
etc, so for a 500,000 sq ft bldg you could have 10 or more Multi-stage pumps risers off a incoming water supply.  The commentary in the above referenced handbook states in part; "The total number of manifold risers on a single header SHOULD be limited so that in the event of an impairment to the supply piping, the total area impaired is limited." Now don't confuse this with a riser "protection area" because that gets into whether it is dry or preaction, racks, etc.
As long as the fire line service is sufficient to supply the worse system there isn't any limit, as long as it is with in the same structure.

MORE NEWS