At this time I have about 7 quests completed, but some other users I know have 3 or 4 times more quests than I do. I'm not saying that I'm jealous, but I was just wondering so I could find a goo number of quests to complete and be satisfied with.
Well, thats all up to you. If you think you dont need more (beside having the badge (and maybe sometimes some kind of "competitive system" there is no reward), then its ok. If you think you need more... get some^^
There's never too many quests ^_^, but if you don't have an obsession with completing ALL of them i'd say it's enough to have those that don't appear impossible or too frustrating for you to get (like the 100mil combo quest on epic combo redux which practically requires you to leave your computer on doing nothing but counting up for 10-30 HOURS! non-stop)
When i started AG, I got quests for EVERYTHING i could find, but then I started to only get quests from games i liked. It's really up to you how you spend your quest-collecting time, but it helps to enjoy it.
I think it all depends on the intentions. In one case, it would be: If someone likes games with quests he will play them, if he does not like them he will not play them. Otherwise it would be like this: No matter what, someone will haunt all possible quests. Which is quite fun
I love quests, I think they are a real refreshment and a very good entertainment by the way.