From a client perspective, assuming you are talking about DBD::Oracle, there's no arbitrary limit on concurrent connections. There are system-imposed limits on number of file descriptors, etc, but that's not a perl question.

There are settable limits on client connections in the Oracle database, but again, not a perl question.

As far as locking goes, again, not a perl question. Oracle supports locking and transactions, and the DBI interface inherits these capabilities.