Since its pure aloha. So max Bandwidth utilization is 18% of 48Kbps.
And its given, Each station transmits
1024 bits in 50 seconds
$\therefore$ In 1 second --- $\frac{1024}{50}$ bits
Let's say there are N stations.
So N*$\frac{1024}{50}$=18% of 48*1000
N=$\left \lfloor 421.875 \right \rfloor$
$\therefore$ N=421