The shutdown command uses minutes, not seconds, so you should be safe from overflows either way.
With the shutdown command, you can either specify the time as HH:MM (giving the clock time, not the length of time to wait), or as +690 to mean “wait 690 minutes from now”
The other command you're looking for is probably head, although it defaults to 20 lines.
cat somelongfile | head -n 10
will do the job. There's also a similar tail command that outputs the last lines.