let's say that i sold a system to customer A that has 5
indoorMotionDetectors and 3 peimetereSensors. How do i define the
chosen multiplicities in the instance\part? which diagram can i use.

now let's say that i want to create a sub-class called
inexpensiveAlarmSystem. the inexpensiveAlarmSystem inherets from
alarmSystem, but the multiplicity of the perimeter sensors is set to 0
for all instances. is this possible? how do i do this?

you define you system with all variants in a block definition diagram.
A concrete configuration could be shown in an internal block diagram.

That the inexpensiveAlarmSystem has no perimeters could also be shown
in an internal block diagram. Alternatively you could redefine the
property
from the base block.

Hope that helps.

 Tim