Many generators can supply a bigger load than one machine by itself.
Having many generators increases the reliability of the power system, since failure of any one of them does not cause a total power loss to the load.
Having many generators operating in parallel allows one or more of them to be removed for shutdown and preventive maintenance.
If only one generator is used and it is not operating at near full load, then it will be relatively inefficient. But with many machines it is possible to operate only a fraction of them. The ones that do operate near full load and therefore more efficiently.