MPI Matrix Multiplication - Task sharing when size(of procs) < rows of matrix

I am trying to perform matrix-matrix multiplication in MPI using c++.

I have coded for the cases where number_of_processes = number_of_rows_of_matrix_A (so that rows of matrix_A is sent across all processes and matrix_B is Broadcasted to all processes to perform subset calculation and they are sent back to root process for accumulation of all results into Matrix_C) and I have also coded for the case when number_of_processes > number_of_rows_of_Matrix_A

I have no idea how to approach for the case when number_of_processes < rows_of_matrix_A.

Lets say I have 4 processes and 8 * 8 matrix_A and matrix_B. I can easily allocate first 4 rows to respective ranks of processes, i.e 0,1,2,3. How should I allocate the remaining rows so that I wont mess up with synchronization of the results which I get from respective processes.

Side note of my implementation: I have used only MPI_Recv, MPI_Send for all the coding part which I have done.

Thanks in advance.