to my understanding, there is no technical limit.
Though I believe to remember that it used to be ~16.000 some time in past.
Class SYS.Database maps to ^SYS("CONFIG","IRIS","Databases",<DBNAME>) and has NO limit there
similar Namespaces are stored in SYS("CONFIG","IRIS","Namespaces",<NSPCE>) an are covered by %SYS.Namespace
If there is any limit it must be related to internal memory structures. (gmheap ??)