There is no threshold for syslog server count that I know of.
I have worked with cusotmers using upto 8 syslog servers.
Since it is going to be addition of one to your existing setup, I do not see any issues.
Only thing to consider is not to have any any configuration when you configure multiple servers(say 8/10).
on having configured any any, during heavy traffic, logging could potentially result in high CPU.
You need to seperate system and security logging.
How to configure system and traffic logging:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB16634
Regards,
Raveen