This is kind of like asking "how long is a string" 
because it just depends on how you need your application to work. You would be the best person to make this determination.In most cases, we have a unique key for each particular action. However, if there is a permission that is somewhat repetitive and will be used for the same purpose over and over again, and more importantly, has the same meaning in each of the situations it will be used, then we may share the permission. I just depends on how granular you want your security to be within your application.