I hope people don't add fireball into their "active abilities" list ^^. Let's analyze a common DA2 mage build.
A common optimized nuker mage would end up getting
4 actives from elemental (ball, firestorm, grasp, cone) 8 talents
3 actives from spirit (bolt, bomb, filler dispel to get the mastery) 8 talents
2 from blood (sacrifice, wound) 5 talents
4 from force (push, pull, maker fist, ring) 6 talents
3 from arcane (blast, barrier, prison) 5 talents
That's 16 abilities and 32 talents that you can barely get by the end of the game at level 28 with all the extra talents from tomes and arcane emporium.
Of them, fireball, push, blast, maker fist, dispel are either completely useless, or become obsolete the moment you get the better abilities. This leaves us with 11 worthwhile by midgame, which of them, arcane bolt and wintergrasp become situational by the time the staff dps starts becoming respectful. This brings it down to 9 useful and 2 situational (wintergrasp to trigger a brittle for varric to take advantage of, spirit bolt for disorients.
You can also invest on horror early and respec out of it with a maker's sigh earlier. It's good for some encounters in the very early game.
Essentially the gamestyle from there goes ring->pull->firestorm->walking bomb->wound, with cone as reset, barrier on standby, prison on staggers or elite melee damage dealers and sacrifice to control the healthpool.
Mage is the most ability intensive character. Rogue and warrior can literally get away with 5 abilities. decoy, marked for death and the 3 nukes, while warrior assault , scatter, sacrificial, barrage, cleave (and perhaps a devour, not too important if the party is good though). It's not too far fetched to design the system for 8 active abilities max.
Sure these are very optimized builds. For example a lot of people may want to play templar 2h warrior that has many more active abilities, or a mage may want to forgo spirit mastery and invest in primal or entropy, or don't want to upgrade barrier, which increases the number of active abilities he can get. And that's fine. But keep in mind that the mage build becomes complete at a VERY late stage that a lot of people won't even reach. He always has enough spells to play, but getting a better spell makes the old ones obsolete. That's healthy. Just because you can get 50 abilities, doesn't mean that all of them count as far as design is concerned.
Personally I don't like fillers till I get the later stronger abilities. I prefer to fill the void between acquiring these abilities with passives and specialize the gear to cover the weakness of not having a complete build.