Maximum MAVlink instances

In “ardupilot/libraries/GCS_MAVLink/GCS_MAVLink.h” there’s definition of how many ports may have MAVlink protocol.

#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
// allow extra mavlink channels in SITL for:
//    Vicon
#define MAVLINK_COMM_NUM_BUFFERS 6
#else
// allow five telemetry ports
#define MAVLINK_COMM_NUM_BUFFERS 5
#endif

What’s the reason behind this limitation? I need MAVlink on all available serials and USB on my Pixhawk 2.1 The Cube, i.e. 6 ports. What will happen if I built it with 6 ports?

Is this some kind of legacy from 1MB or even APM times?