My pc specs are i5 6500, 8GB ddr4 and a gtx 960 2 gb on modpacks like sevtech and exoria my game gets fps drops and go's un-resposive
Probably ram usage, the suggested amount of ram for sevtech is 8gb, which would be all of your ram (And it's generally recommended not to allocate all of your ram). 2gb is nowhere near enough for large packs.
So if i add another 8 gb i should be able to run any modpack that i want?
You need to allocate the game more RAM. Use google to find out how to do so with whatever launcher you're using.