openvpn incoming connection limit

Hello everyone,
I’m running an OpenVPN server on a RB800 (ROS v4.16) using MD5/AES192.
One of the requirements is that it should terminate 120+ users simultaneously.
For (stress) testing purposes, I configured a couple of RB450’s as OpenVPN clients, each terminating a number of accounts on the RB800, so the number of active accounts on RB800 reached 125.
The problem I’m having is that RB800 doesn’t authorize/authenticate more than 5 incoming connections at the same time, with log entries:

no more listening for incoming connections: too busy

That process lasts about 8 minutes (until all accounts are logged in), while the CPU usage on the RB800 runs approximately at 25~50% with rare peaks up to 100%. After all connections are up, the CPU returns to “normal”.
Is there a way to increase that 5 incoming connections limit, or running more than 1 instance of OpenVPN server simultaneously?

Tnx.

I’m fairly sure it’s not an OpenVPN issue, but probably a questionable anti-DOS RouterOS implementation.
Any ideas for a (no-metarouter) workaround?

Tnx.