There is no set format for jazz.  The size of the ensemble may vary from a pianist to many instrumentalists.  The music can take many forms, a simple tune with light ad lib by the lead players to extreme modern jazz with unusual chord patterns and styles. 

Jazz musicians are great exponents of their instruments and the style itself and it can take many years to master and create your own style in the field of jazz music.

There are different types of jazz music and the common ones in use in Army Music are:

• Dinner or background jazz
• Trad Jazz

Dinner Jazz is often a small group who take a theme for a popular piece of music and manipulate the content.  Often switching the solo line between different instruments or in some bands cases vocalists.

The extension of this in terms of ensemble size is the Big Band.

Trad Jazz is at it says.  Traditional jazz music born out the the US town of New Orleans.  The structure of this type of jazz is more rigid but none the less impressive

A jazz ensemble is ideal for less formal occasions or perhaps background music.