I'm typing this before reading everyone else's responses so that I can give my opinion uninfluenced by others. Then I will read everybody else's.
Also, I'm not just going to talk about the number of episode. I'm going to go into more detail about how I think the episodes should go and what should happen in them.
Let's say Ash wins the League in Gen Q. Here's how the Gen Q league should go:
Episode 1: about getting and training reserves (like An Old Family Blend!)
Episode 2: Ash battles a 3v3 against a trainer he's never met before, like Jeanette Fisher, Macy, and Katie (I know Katie was a 6v6, but I just can't think of any other names right now of trainers Ash had never met before their league battle)
Episodes 3&4: quarterfinal full battle against a trainer he met a short time before the league, but that trainer was not a major rival (like Morrison or Cameron). Reserves should be used; Totodile should evolve in this battle.
Episode 5: quarterfinal full battle shown in one episode between two of Ash's rivals: this should be the Paul/Barry, Cameron/Bianca, Sawyer/Tierno
Episodes 6&7: semifinal full battle against a minor rival (like Nando, Conway, or Stephan) except i would prefer her to be female
Reserves should be used; PRIMEAPE should appear in this battle
Episode 8: similar to Kalos league episode 5, there is a break between the semifinal and final. This time, it should be used for Delia, Professor Oak, Misty, Brock, May, Max, Dawn, Iris, Cilan, Serena, Bonnie, Clemont, Lillie, Mallow, Lana, Sophocles, Kiawe, and all of Ash's traveling companions between Gen VII and Gen Q to arrive to watch Ash in the final.
Episodes 9,10,11: final full battle using ONLY his Gen Q Pokémon against his Gen Q main rival
Episodes 12&13: first member of the Elite Four, full battle that Ash loses,
using Pikachu, Charizard, Sceptile, Sunglasses Krookodile, Ash-Greninja, and Mega Heracross