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Use the 'wc' command:
wc myfile[123].txt
find -type f | wc -l
PS -e|cut -d " " -fname|wc -l
tail -10 anyfile | wc
The 'uptime' command will tell you exactly how many users are on the system. There are other variations of this, including counting the number of words from the 'users' command, etc., but this is the easiest.