How to avoid too much running apps? Meanwhile I have many running apps in my phone and I found that a lot of apps keep running because they "trick". They register as a boot receiver so they start with the phone, then register as a "new app installed" receiver and so on. The do everything to keep running. If you kill them, some of them start again because the creat a service which checks if they are running.
I guess these apps do that because they want to transmit information about the user or phone.

What can I do against those apps when the phone is not rooted? The task killers that I checked do not work. I think they are fake.

(With a rooted phone the Autorun manager seems to work good)
Perhaps you may want to look into hibernating the applications as opposed to killing them with a task-killer.

The former is advantageous, since it takes care of all background services of the given app and prevent them from waking up, until such a time when the user executes them.

One candidate in mind perhaps the popular one is Greenify. One XDA user(lioux)  has been generous enough to provide a detailed mechanism of Greenify;


  It (Greenify) is a GUI (graphical interface) that lets you select applications
  that you don't want running on the background, starting automatically
  when the phone boots or responding to intents.
  
  It provides useful data
  about running applications so that you may make an informed decision.
  It uses the android command am to stop an application. It will no
  longer start during phone boot, respond to registered intents or be
  triggered by alarms from the system:


 am force-stop package.name
 am start package.name



  Everything it does is temporary. The am command does not touch the
  installed files or any configuration information. It merely tells the
  system to "forget" about the application.
  
  The system re-learns some of the intents again the next time the
  application is executed: launched by the user or launched by another
  application.
  
  If the application does not behave as expected after being awakened,
  remove it from Greenify then reboot your phone. 
  
  Some applications do
  not re-register events when executed. Sometimes, they only do it at
  boot. Furthermore, applications might have specific "relationships"
  that might be disturbed when one is stopped (location service for
  instance).




Up to date, Greenify has been constantly updated, to include handy features even for non-rooted device (although the functionality is limited compared to using it on rooted device and even much better when integrated with the Xposed module)


  Most of the basic features of Greenify work out of the box on
  non-root devices, still some useful additional features require a
  one-time privilege escalation procedure different than normal
  permissions, which is enforced by Android system, using adb


For that see: How to grant permissions required by some features on non-root device?

Note: Some users have observed some certain functions are still limited despite granting permission through adb (may possibly be a bug in experimental releases)

Acknowledgements


Greenify
How to grant permissions required by some features on non-root device?
Greenify with ADB permissions?