You're assuming it means that only 7 things can be going on, thats not the case. The iPhone will "multitask" only what needs it and the only limits are available RAM and CPU. Theoretically you could have more than 7 tasks running in the background.
The 7 APIs are the "types" of tasks that can be run in the background. Any app can use 1 or more of the 7 APIs including background audio or task completion like a file upload / download to name 2 of them.