A story should be as long as it needs to be. If you can tell a whole story with eight sentences, fine, if it needs to be longer, it needs to be longer.
There is a whole type of fiction called Flash or Sudden fiction. Sudden Fiction stories are fewer than 1000 words long, but there are sub-sets within this that include drabble (exactly 100 words), dribble (50 words) and 55 and 60 word stories. I haven't come across a type that has a set number of sentences though.
Telling stories in so few words is a real art form, and a lot of fun. Come and check out the
Suddenly Fiction forum some time.
