In the first four seasons (the first era of My Little Pony: Friendship is Magic), most of the Mane Six each had about an average of 1.5 absences in one half of a season (13 episodes), depending on the season.
Then, as the second era began with Season 5, they got more absences, with Fluttershy literally being the only one to have as much as five absences in one half of each of Seasons 5 and 6. But now, it's getting even worse still.
What do I mean by this? Well, let's compare between the two most recent seasons of the show:
In Season 6, Rarity made the most episode appearances with 15 out of 17 episodes; Twilight and Rainbow each had 14, Applejack had 13, Pinkie had 12, and Fluttershy and Spike were both tied in last with 11 appearances. But in Season 7, not ONE single character has reached that amount of appearances after 17 episodes. Not even any of the Mane Six!
Really, I don't know what is going on with those crazy writers. I sure hope they improve their skills and give more appearances to our favorite pony friends unlike in this season.