Though one would be sufficient, it is recommended that you have several anti-spyware program in your computer (probably 2 or 3)... Because there might be a case where a spyware is picked up by one scanner but not by the others...
What I have is AVG Anti-Spyware, SpyBot Search & Destroy, Ad-Aware SE and Spyware Terminator.
Also it is good to have CCleaner so taht you can remove any residue that those s[yware remover left behind during removal... It helps clean up all your temporary internet files and many other things and hence making your computer works faster, at least noticeably faster...