This is a hard one for me, sometimes I'll create a character so that I can kill it but then I'll like them and not want to, hehehe, but I'm a softy.
1) It depends on your story. If it's about the happy fun times of talking bunnies, than 1 is too many, if it's about world war II then the sky is the limit.
2) This depends on your message, if you want to portray to the world a bleak and sad outlook on life than kill all of your main characters, however, if you want a good story with some sort of hopeful message than you can still kill as many as you want as long as their deaths offered some sort of balance to the universe or promise for the future, if you don't really want to kill any of them but feel that you're story is too violent for everyone to get out alive then either kill the character you feel will be the most beloved by the readers and his or her death should be enough, or kill a few them who's deaths won't destroy the lives of your other main characters.
3) This is a hard one to sell, I will say that if you are going to kill them all you had better have a damn good reason otherwise the reader will hate it (it's like a story where it turned out to be a dream the whole time, you get to the end and you're like 'what the hell was the point of that?')
4) The only answer to this is that you have to do it when the plot calls for it, don't decide who you want to die and then kill them all in an explosion at one time just to be done with it and give your other characters a reason to mourn, it'll be forced and the reader will know, also, don't decide to kill one character every two chapters just because, it'll be dull and repititive. A good reason to kill them all off at once would be if you need a big event to happen and then you want the rest of your story to be about you surviving characters rebuilding their lives, or something along those lines, a good reason to space them out would be if your characters are going on a series of missions and you want the reader to understand exactly how dangerous these missions are, then maybe every once in a while one of them should die.
There really is no wrong or right answer to these questions, just be true to your plot, characters should not die because you feel like you should kill somebody off but rather you feel like it's unrealistic for everyone to live. Sorry for the rant, I hope this helped!