Unity and Google devs have thought about that when they published the "ads" plugin.
I have read about the ad generation some time ago. If I find the specific link I will edit the post. But unity said in their forum that unity-ads only generates a specific amout of ads per phone. IE 5 per day. After you have watched these ads I'm not sure if new ads are generated or unity stops providing you with ads. Also there is a calculation behind the ad system which takes the amout of devices ads are played on into account. Also the payout decreases the more ads you play to your users.
I'm also pretty sure that such an app is against unitys or at least googles ula.
But thinking about it. I'm not sure if you could turn out a profit with using this method. If you run an app your display must be active the whole time. Also to get the ads you have to connect to wifi or mobile internet for at least some time to recieve the ads and confirm that they were watched completly. So your energy need with 20 phone will be considerable. Also if you get good ads they get you around 8-12$ per 1000 clicks. I'm not sure how unity spreads the ad quality but consider them rare in a "real life idle farmer". So if you can watch 100 ads per day and have to run you phones for about 15min to not get punished to hard (just spittballing here. I don't know how the revenue calculation works in detail) that will be about 80ct -> 1.2$ per day. I don't know how much you pay for electricity but profits will be thin and I think you need to pay for 20 phones first.
TL;DR: Time invested in screwing with google will be wasted. Invest your time in developing a real game or application users will stick with and have a look at the calculations behind google ads.
EDIT: after looking around in the unity forums it seems that achiving constant 8-12$ per 1000 clicks is utopian. So my guess for such an app would be around 1ct per day.