340-600 has 2 bottles per engine (in the pylon) as does the Rolls Powered 747-200/300/400 (in the nose cowl) the CF6 powered 747 has 2 per wing in the L/E, JT9 has 2 per engine as well. 777 has 2 bottles in the cargo bay.
When running engines our SOP is pull the handle, fire the 1st bottle wait 30 secs, if still a fire warning fire the 2nd bottle and then start taking very large steps from the side opposite that that is burning, having previously called ATC for fire engines and shutting the a/c down. Agent is released almost instantaniously.