Fps rate in in editor unity is showing upto 70 but after build in mobile it's showing only 8 , is this normal?
Depends on the application and depends on your mobile. Unless your mobile is equally as powerful as your desktop which runs the editor. Usually it is normal having less fps on mobile. But I already told you that you really shouldn't care about editor FPS, and FPS in general, move to measure milliseconds and budget things.
No idea what the device you're testing it on, but mobile generally needs far more optimization than a PC. Use the profiler if you wanna figure out the issues.
Am I mistaken, or didn't you have a thread on this just yesterday? (Edit: Not quite the same topic, just very close, though @LurkingNinjaDev's answer in there did cover this.) Yes, this is completely normal. Your PC running the Editor is probably far more powerful than your mobile device. A significant part of game development is optimisation, where you tweak and configure and change how things are done so that you get the result you want with minimal work from the hardware. Testing performance in the Editor has next to no value in most circumstances. The Editor does a bunch of stuff that won't be happening in builds, which impacts performance. The Editor almost certainly runs at a different resolution. Data is often loaded in different formats on different platforms. Unless you're making a PC game, it's probably a different type of CPU and GPU. This can make a huge difference even between different PCs, let alone a dev machine compared to an average phone... Check out the Unity Profiler and learn how to use it to measure performance of apps running on your target platform (ie: your mobile device). This is the easiest way to get started at figuring out what's slowing your game down and how to deal with it.
By the way... the "General" are is specifically not a support area. When you've got questions about how to do stuff you should ask in the relevant sub-forum rather than here.