Number of processes needed to maximize CPU utilization under I/O wait conditions
You want all processes to fit in memory. If there are $N$ processes, then together with the OS they take up $256+128N$ megabytes. Since there is only room for $2048$ megabytes, we need $256+128N \leq 2048$. Dividing by $128$, this is $N+2 \leq 16$.