Double tapping the iPhone home button shows a list of recent apps, not running apps. If you have 100 apps installed, and you have opened all 100 apps at some point since the phone was powered on, all 100 will be in the recents list. This is not an indication that 100 apps are running.
Furthermore, apps that are running do not always appear in this list. Force quit Mail or Phone. Do you still get calls and email? Yes, those system apps are always running, even if you remove them from the recents list.
Newstand also appears in the recent apps list, and it's not even an app, Newstand is a folder!
iOS automatically reclaims memory from apps that have not been used recently, causing them to take longer to start up again later. Removing an app from the recents list by swiping up also forces iOS to reclaim the app's memory, which can help for troubleshooting a broken app, but in all other cases is reducing the speed of your device, wasting your time and wasting battery. Why wait for the app to start up next time when your phone could have just instantly resumed it?