There are 2 types of quest in this game, Main Quests & Side Quests
Main quests are your main story arc which your session will revolve around. These types of quest should either relate to the entire party, none of the party, or slightly interest the party. What I'm saying is you shouldn't focus too much on one player or the others will feel insignificant, unless maybe you are just hosting for one player and yourself. Anyways I normally like to put these as guild or adventurer requests, you need a reason for your players to hear about it and be interested. These quests should be big and grandiose, not something you resolve in 1 session. These quests can be grouped together and the dm can assign 1-3 per story. They will always level up the player characters fully. It's also good to add side quests into this mix as it will give your players and idea of progress as well as reward them for progressing through your story. Be sure to reward your players handsomely at the end too.
Side quests are more like the building blocks of the main story, things you have to do before finishing the main quest. They don't fully level up a player on their own but together they can help. Its not good to make these too long, too short, too many or too few. I myself unfortunately don't have a magic number of how many there should be per story, when I did 5 the session was short but when I did 11 the session was too long. Really it depends on the dm. If your still not sure what a side quests is here's an example. For Main Quest: A dark wizard has covered themselves with a shroud a dark magic giving them high magical and physical resistance. The wizard is going on a killing spree an no one can stop them. For the Side Quest: There exist four magical elemental crystals that are said to be able disable any magic when brought together at least once, however each one is a temple guarded by elementals. Ok so not the best example but the idea is that they help the end goal instead of just skipping to it.