Mobile WIFI: how close can mobile devices be placed and still have a functional WIFI connection?
Placing WiFi transmitters in close proximity will cause interference.
However the WiFi protocol is designed to work in difficult conditions and devices continuously modify their behaviour to compensate for intereference.

In your case if your application can handle intermittent and limited throughput then it will probably work.
However if you are expecting 300Mb+ per device then probably not.

Microwave intensity drops as a square of the distance so separating your phones by even a few inches will help.

Also (assuming the phones are fairly modern) alternating between 5 and 2.4GHz will help.
Placing each phone in your stack on a different channel may help but since the other phones will effectively raise the background noise anyway it will probably not make much difference.(assuming you can have multiple APs).

If you do have multiple access points then ensure they are distributed around the perimeter and not piled into their own stack as that will just compound the issue.

Bottom line is you will need to test as this is not a common scenario.