Characters were counted by hand based on UESP quest writeups. Characters were only counted if they were questgivers or involved in multiple quests. Only characters from the main quest or faction questlines were counted. Any expansions or DLCs have not been included.
Due to the incomplete documentation for ESO quests, that game probably has more margin of error than others, though it should be balanced out due to how many NPCs were counted overall. I realize that this is an imperfect process, especially considering the very different ways that each game handles quests. I think the overall patterns hold, though, even if the percentages might be off a few points were someone to repeat the process.
You’ll also notice that Morrowind, Oblivion, and ESO have two main quest graphs. The latter is for including characters who are also encountered in the other parts of the game. For Morrowind this is questlines where you must speak to all the house leaders to become Hortator, in Oblivion it is the Aid for Bruma questline where you must speak to the counts/countesses to gain their support, and in ESO this is the Weight of Three Crowns quest where the faction leaders convene on Stirk. Daggerfall, meanwhile, randomizes most of its quest, and the overall graph counts the main quest and nobles quests.
Sample sizes are as follows: Daggerfall (23 total, 10 main quest), Morrowind (82 total, 16 main quest, 34 with hortator), Oblivion (36 total, 9 main quest, 15 with Bruma allies), Skyrim (59, 11 main quest), ESO (278 total, 6 main quest, 10 with Stirk).
Conclusions and interpretations under the cut.