Concurrent sessions for Balance 310

Hi there PEPLINK Gurus,
I want to ask whats the maximum concurrent sessions (NAT) can Balance 310 handle?
My balance 210 always hangs up whenever sessions reaches 50,000 although I only have 60 users attached to the LAN. I need to reboot the device, as DNS failure always occur on the 2 WANs (taking turns on DNS Failure)
thank you