What is the reason for a 9 gun salute at a funeral?

The amount of firing will vary. If performed near ongoing battlefields, and not enough men are available, there might be less than the number of men required in the firing party. If no blanks are available, service rounds (live rounds) will be required. If there's an ammo shortage, less rounds will be expended. If there's a safety concern over live firing, again, a shortened ceremony. Gun salutes originated out of necessity; Burning gunpowder is expensive, especially when large amounts are loaded into cannons. Up until the US Civil War (1861-1865), (the US Civil War was transitional) Infantry Rifles and Guns (Cannons) were mostly muzzle loaders. When warships entered ports (practically any port, home or foreign) for safety reasons they had to UNLOAD their guns (cannons). Muzzle loaders can only be unloaded by firing them. After the warship fired it's guns, the Port's Military Garrison (Fort) would acknowlege the warship by firing it's cannon. A salute is an acknowledgement. Gunfire is a military honor. The dead were honored the same way. When a soldier is executed he is shot by a firing squad. A man with less honor will normally be hanged. Although a squad is normally 12 men, a patrol is normally 6 to 7 men (or six men and a leader). For safety within controlled areas containing ceremony, a patrol of men (6 or 7) is easier and safer to command and control with firearms. The traditional three cheers for a departing "brother in Arms" is fired by three volleys.