20 is a little of an exaggeration, but that is right. Vampirism, Corprus, Cure Corprus, Fortify Maximum Magicka, Restore Magicka, Stunted Magicka, Sun Damage, stupid stuff like Weakness to Blight Disease, etc.
I don't see how you're getting such a high number (exaggerations aside) for Skyrim. Morrowind HAS MORE SPELL COMBOS BECAUSE YOU CAN MAKE SPELLS THAT USE MANY SPELL EFFECTS AT ONCE. The MAXIMUM you can use AT ONCE in Skyrim is 2 different spells, and if you want to go ahead and count their 5 or so different states, that's 10 different potential combos, of course, then you have to count the same for Morrowind.
2 < 10+
Stunted Magicka, Sun Damage, Vampirism, Restore Magicka effects exist in the editor in Oblivion as diseases, effects or potions.
More of a concern, yes you can have two spells active at once, has benefits from Oblivion and Morrowind single spell, mostly weakness/ mind control in one hand and damage in the other, however an combined spell has multiple benefits, add soultrap to a spell and use it to finish an enemy, add light or a minor effect who times out 2 second before the turn undead or mind control spell does, so you can cast it again.
Now the number of useful spells in Morrowind and Oblivion: 100 strengths *100 durations * 30 areas of effect * 2 touch or target. 600.000 potential single effect fire damage spells.
Yes it's more than you need, however I regularly used four effect spells in Oblivion silence+ fire damage+ weakness to fire+ weakness to magic.
Yes you have the same for shock and with different strengths, some has area effect on weakness.
or summon fire elemental+ long time low level heal self + invisibility. Found some healing is nice on a getaway spell.
Again duration of invisibility, heal and summon and strength of heal.