Apps & Galaxy 6's rate of battery life decline
Having more apps installed to your device storage will not at all affect your battery life, unless perhaps your storage is nearly full and the OS / storage controller needs to spend extra time finding free blocks.

Running more apps will certainly affect your battery life.  Many apps will use services, or register broadcast receivers that get events and run a task, etc. thus using CPU time and battery.  However, this is absolutely not different from other mobile OSes.