What is the maximum amount of an Elements asset that can be issued?
There is no maximum that can be created, however you may run into issues with RPC-APIs in Elements if any wallet contains more than 92,233,720,368.54775807 at any time.
Confidential transactions will function with any realistic amount (you would have to re-issue an asset more times than possible to overload the 256-bit confidential amount type, however the receivers wallet would fail if int64 overflows.