How many magicians quit after the 7-year time skip?
Yes, after the "Tenrou" arc, most of the Fairy Tail members quit the guild, but all the members who had screen time are still in Fairy Tail .

Most of the important characters, or characters who had major or average screen time and were at "Tenrou" island (Natsu, Ersa, Lucy, Gajil, etc.), and even the most minor characters who had less screen time (like Waren, Romeo, Romeo's father, etc.) are still in Fairy Tail.
Even if some characters are missing (I personally didn't notice any), all the important ones are still in Fairy Tail even after "Tenrou" arc. On the plus side, we even get a new character - Kinana.

My answer is based on only the anime, not the manga.