Hello Alexander
You need to set "Maximum Concurrent Jobs" in these directives below:
===== Server side =====
## bacula-dir.conf ##
Director, Storage, Client
## bacula-sd.conf ##
Device, Storage
## bacula-fd.conf ##
FileDaemon
===== Client side =====
## bacula-fd.conf ##
FileDaemon
You are using an older version, is highly recommended to update to 7.4.0.