cgi101.com is a very quick cgi reference.. you can also look at:
http://www.jmarshall.com/easy/cgi/

for more comprehensive perl stuff maybe: http://www.codebits.com/p5be/ch10.cfm or wdvl.com

max clients really depends on your server/bandwidth.. if you want to increase the speed of your perl look into mod_perl, wdvl has some decent articles on the subject..