My question is - how many ad networks can be integrated in one app? Or, at least how many are recommended to be integrated in one app? Is there a best practice for this?
I am currently using AdMob, but I started creating accounts on AppFlood, AdFonic, etc.
I have searched the forum, but I haven’t found any relevant answers for my question.
Thank you! 
You can integrate as many but best would be two add only 1 EULA ad network. For banners and inters, its ok as long as you can send significant traffic to each one of them to generate revenue.
I don’t think there is a limit to how many ad networks you can add into your app. But be aware that showing a lot of ads to users might get them annoyed and they will end up uninstalling / giving a bad rating. Also, some ad networks require extra permissions which some users dislike.
Thank you for your replies and for the help.
Newbie question - Is featuring an EULA for/about ad-networks compulsory? If yes, should I create my own or do ad-networks provide them, or can I somehow integrate information in my existing EULA?
**mention - I have a standard EULA for the app - is there anything I should mention in it about ads?
Thank you again!
You only need to show an EULA if you are going to have ad units outside of the app. Any network that has these units will supply the EULA to you anyway.
Obviously you can always include in-app ads in your custom EULA if you have one, but its not required.
Thank you for the detailed reply! 
I really appreciate it!