The documentation for 12.2 up says the limit is 4098...
Is the db cloud hosted on AWS?
There's a couple of Oracle Doc's about this, and a possible work around:
mentions a bug 26263110 apparently fixed in 18.1 might be relevant as you are on 18.0
...and Doc ID 2315818.1 12.2 and later may also apply.
Raise an SR with MOS, they may suggest you modify the _ paramateter mentioned in Doc ID 2538172.1
This database is on AWS, so I think we are hitting the bug mentioned in MOS 2538172.1. I tried setting the underscore parameter _cdb_disable_pdb_limit=true and was able to create additional PDBs.
Thanks for the help.