Unless units fight in melee combat there is always a spacing of N meters between them. Virtually, there is no way for two friendly units to collide with each other, while enemy units can engage or disengage to/from combat only by performing special movement maneuvers called Charge and Retreat correspondingly. There is a bug in DO when enemy units may still collide with each other during wheeling.

However, sometimes there may be situations when rival units collide with each other during wheel maneuver

Model size and contact

Model size determines how many models can be in contact with one model 
There are 2 model size types in DO: regular and large
regular-size model is any model from human to horsemen and it can be surrounded up to 4 regular-size models and up to ? large-size models
large-size model is usually a monster like troll or giant scorpion and can be surrounded up to 8 regular-size models and up to ? large-size models

This video demonstrates 1 large-size model vs 8 normal-size model contact 

Unit1 > Unit2 attacking -  up to +4 models bonus
Unit1 > Unit2 defending - up to +2 models bonus

Unit1 = Unit2 attacking -  up to +2 models bonus

There is a limit of maximum allowed models in contact which depends on such conditions as:

• both units size
• who charged (attacker)

charged arcs and unit formation doesn't give models in contact bonus