You mean how many visitor visits a post on your site in a day? If you just need the stats, you can try Google Analytics. If you want to display the information to usesr. Depends on your programming skills. If you know database and programming language, you can make your own counter. Or if your programming skill is limited, you can use something like www.easycounter.com. Which can display your visitor stats to public. I am not familiar with phpBB, but they may have free mods out there that can handle stats.