Question

why there are only seven crystal systems??? why not other number

Answer 1

What other type would you use? There are 7 systems, but 32 different point groups; there is a limit to how many different systems you can make; to understand this difference you have to study geometry and specifically point systems to see why there are only 7 systems.

Answer 2

there r more than 7 crystal systems bt they are not symmetric enough to study

Answer 3

yaa whatever u told is right !!!
but still i have not convinced i need some scientific basic explanation for this
and even why only simple cubic ,body centere , face centered , end centered , why not edge centerd is possible