A Dragon Master, also known as a Dragon Lord, has a way with summoning lesser Dragons.

The following guys will arrive in greater numbers when summoned by a Dragon Master:

1. The Fire Drake (2, 6 for 1, has Fire Breath)
2. The Ice Drake (2, 7 for 1, has Frost Breath)
3. The Swamp Drake (21, 7 for 1, has Poison Breath)
4. The Cave Drake (2, 4 for 1, has Darkvision and very high Protection)
5. The Wyvern (2, 3 for 2, has Wings and Discipline)
6. The Sea Serpent (2, 4 for 1, is Aquatic but causes Fear)

That's all of them; "greater" Dragons such as the Tarrasque and the Tatsu are too full of themselves to acknowledge a Dragon Master as their sovereign. The above guys are all summoned through Level 4 Conjuration Rituals.

Dragon Pretender Chasses typically have Dragon Master (1), so they get one extra Dragon per summon. The Dragon Master Ritual (Enchantment 8, 3, 30) permanently gives Dragon Master (2) to its caster, which doesn't stack with itself but still stacks with other sources. The Dragon Sceptre Item also adds Dragon Master (1).