Should I use different AdMob Ad Units ID in different Activities?

You can use one Ad Unit ID for all banner ads within your app and a different Ad Unit ID for all interstitial ads within the same app.

Thus, if your app contains only banner ads, it can have only one Ad Unit ID. If your app contains banner and interstitial ads, it can have two Ad Unit IDs.

However, according to the official admob guide, YES! you can use one ID per activity:

An Ad Unit ID is a unique identifier given to the places in your app where ads are displayed. If you have an app with two activities, for example, each displaying a banner, you need two ad units, each with its own ID.

For reporting sake, use different ad unit for each application. This helps you know how each application is performing in regards to eCPMs, requests and clicks. It will help you understand the market, adjust or improve your apps as needed.

It's not against the policy if that's what you want. (Check this thread)

Actually, according to the Admob for Android Guide , yes you should!

An Ad Unit ID is a unique identifier given to the places in your app where ads are displayed. If you have an app with two activities, for example, each displaying a banner, you need two ad units, each with its own ID. 

I had the same concern and looked into the link Nana gave. This is one of the messages that caught my eye:

Hi Sonic,

You're still allowed to do this, but we recommend against it, and if you link an app then house ads might not work. We recommend creating a different Ad Unit ID for every placement you have (which means you'll probably have multiple per app).

Hope this helps!

Amy Quispe

IMHO, you should have a different ad unit for each BannerView you display.