Single throttle bodies are more cost effective to produce hence usually found on most road going engines. Individual throttle bodies can be found on many performance cars and most Naturally aspirated competition cars. The reason is they offer large improvements in throttle response and efficiency due to the reductions in pumping losses from atmospheric pressure being available for each cylinder rather than being forced to take a longer path through the inlet manifold runners. This is achieved by applying an individual butterfly for each cylinder rather than all cylinders sharing a single larger throttle body.
The result is a change in engine note and increase in volume whilst improving overall power levels and throttle response.