1.  The basic idea of most severance (separation) agreements is that you will give up any legal claims in return for payment of some money.  If you don't have any possible legal claims, then you generally don't have any leverage to ask for more money.  If you do have some possible legal claims, then you might try to negotiate a better deal.

2.  There's no easy answer to the question of how much severance is appropriate.  Many employers do not offer any severance.  Some employers that offer severance to all departing employees have standard formulas.  For example, a company might offer one week of severance for every year of employment.  So, if you worked at that company for five years, you would be offered five weeks of severance.

3.  About severance agreements and unemployment benefits.