After the time skip following the Tenrou Island arc, a lot of Fairy Tail members quit. So, my question is: how many members did Fairy Tail have before the time skip, and how many do they have after the time skip? How many of them quit?
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.
I don't think there is an exact number, but I know episode 96 would have been the the last episode where you can see other characters that were in Fairy Tail.
In 13:10 of the link above, I have not seen these characters after the 7 year gap.
Also there are other background characters throughout this episode.
I couldn't find the exact number right before they went to Tenrou Island but we know how many member they have on Battle of Fairy Tail arc. When the battle of Fairy Tail begin there was 83 member battling each other
This excluding Makarov, Happy, Laxus, the Thunder God Tribe, the member who were out on mission such as Gildarts, Mest and Mystogan (although Mystogan joined the battle latter) and the member inside the guild that turn into a stone such as Erza, Lucy, Cana, Juvia, Mirajane, Levi, and Visca. So if we count those guys, there was 99 members of Fairy Tail that time (not counting unnamed member who were out on mission and those that joined after Battle of Fairy Tail arc such as Wendy, Carla, and Pantherlily).
Based on Fairy Tail timeline, battle of Fairy Tail happened in October 15, X784, and S-Class Mage Promotion Trial happened in December 15, X784. So only two months has passed. In those two months, Laxus was excommunicated from the guild, and Mystogan went back to Edolas, Lissana also back to Fairy Tail while Wendy, Carla, and Pantherlily joined Fairy Tail. With that in mind, there was probably around 100 members of Fairy Tail on Tenrou Island arc.
Seven years later, we know Fairy Tail only have 35 members, 20 of which went missing on Tenrou Island. 12 old member who stayed on guild, they were Makao, Wakaba, Bisca, Alzack, Max, Nab, Warren, Laki, Vijeeter, Jet, Droy, and Reedus. Two new member, Romeo and Kinana. And Mest. Excluding Laxus who were still excommunicated from the guild that time. So there was around 60-70 members quit Fairy Tail in 7 years