Amdahl's law is mainly used to predict the theoretical maximum speedup for program processing using multiple processors.

Amdahl’s law states that in parallelization, if f is the proportion of a system or program that can be made parallel, and 1-f is the proportion that remains serial, then the maximum speedup that can be achieved using n number of processors is 1/((1-f)+(f/n)

If n tends to infinity then the maximum speedup tends to 1/(1-f)

Increasing frequency will get to a point of saturation where any further increase will not affect the speed, therefore you will have to rely on other methods to increase speedup hence multicore processors are preferred over single core.