From the data available the speed of the lifts is crucial to decide the no of lifts for a given no of apartments.
If the speed of the 2 lifts are above 1.25 m/sec then this will be sufficient for 100 nos 3/ 4 bed apartments. It will not make sence to provide more lifts since the annual cosst keep escalating and may need to shut down one lift permanantly for optimum use. If so, This Builder has got good Technical knowledge .