The amount of people who can join depends on your Rally Capacity. While certain buffs are shared amongst all that join, the biggest influence is the base rally capacity of the one who starts the rally.
Try to have the person with the largest Rally Capacity start the rally. This ensures that the most troops can join.
You can increase your Rally Capacity by upgrading your Hall of War.
You can increase you March Capacity by upgrading your Stronghold, researching it under Combat at your University, or (when your Dragon accompanies your Troops) by upgrading/enhancing and assigning your March of FuryDragon Skill.