My legs and arms will get numb sometimes when i don't move for while. why is that?

Various causes.: Perhaps most common is pressure on one or more nerve roots. One should see ones doctor or a neurologist to assess potential compression of one or more nerve roots, often at the level of the spine.

See a neurologist: The possibilities include peripheral neuropathy or pinched nerves in neck or low back. Diabetes is the commonest cause of neuropathy.