I haven't played the new faction challenge, but in the old one the last quest was repeatable, which required 5 wins. So, number of quests is basically the number of wins for everyone that went +5.
The last quest is repeatable, but i dont think it has anything to do with wins, i think its always 'do a daily quest', to ensure players cant do too many faction challenge quests at a time. Besides, plenty of players dont play that much and might only do the first few quests, which are faction specific and usually require some deckbuilding conditions.