Kiting

[Malong]

But seriously. Playing alone or with friend makes game too easy. Less nords spawn and it's faster to reach wave 15.

I guess, this mod was created with cooperation in mind.

As NordInvasion page says:

The Nords have invaded Swadia and His Royal Highness has summoned all his forces to defend the nation and repel the invaders. Reports are coming in that huge armies are making their way through the pass at Jelbegi. Meanwhile, hundreds of refugees have been making the arduous trip to safety following attacks on local villages.

The King has called for all available units to assist.

"summoned all his forces to defend the nation" not "summoned his loyal soldier to alone deal with Nords"

Except 16 players is as arbitrary and as good as any other smaller natural number. The entire "nation," isn't capable of getting on a server anyway:

Soldiers in the army 102,364
Soldiers on the battlefield 51

Either way, it's just flavor for roleplaying and/or immersion. If you want to actually stick to the story, you should never respawn, use out of game communication, type when dead, and so on.

And has it been proven to be easier "alone or with a friend?" The fact that few people can successful complete Ragnarok in a small group seems to show otherwise. Meanwhile it's trivial with 16 players despite there being more bots.