to my knowledge there is no explicit limit, but there very may well be practical limits.
We have been working on this part of the build process and also the logging for a little while now (you can see how the logging error changed from “System Error” to “build exited unexpectedly”.
There is another person here in the forums who also is trying to upload the same high amount of files (in the tens of thousands) at the same time:
(ignore the first half of the thread until we changed his build timeout)
and my best guess is that something with these large volume of files is causing problems, which is why I need to go figure out who to talk to about it (and then find time to troubleshoot).
One question I do have: what kind of deployment workflow are you using? are you just pushing to the repo and then deploying via CI?
I wonder if it would work better if you used the CLI 
it is worth a try if you have the inclination to experiment a bit.
I know this is super frustrating but I promise that we are actively looking at it, it is just proving more complicated to debug than initially expected, and before we can roll out a fix for these unique cases, we need to test and make sure it is safe to use at scale for 800000+ people worldwide…