From A Wiki of Ice and Fire
Khalasars come in various sizes, the more powerful and feared the Khal, the larger his khalasar. Before the age of thirty Khal Drogo led an enormous khalasar 40,000 strong.
Khalasars are broken into groups, called khas, which are each led by one of the khal's captains, called kos. When a khal dies or becomes to weak to lead, a new khal may take control of the khalasar or the khas may break away and form new khalasars led by their former kos.
The Dothraki will not follow a child khalakka.  Instead the kos will fight among themselves to take the place of the fallen khal and the khalakka will be killed so that he does not grow up to become a rival. 
|“||Death rides with every khalasar. ||”|
|“||Fall upon one of these khalasars and destroy it, and you may find that the Dothraki are not so quick to cross the Rhoyne. ||”|