Malchik originally suggested 50 onions but I said that was way too much.

I might reduce it to 15 though if they're really that hard to find.
Accordin to the UESP Wiki there are 70 respawning onions scattered around in homes and caves, castles and ruins throughout Cyrodiil—although it lists only 59 :rolleyes: Since obtaining most of these would be stealing, many players may have an ethical conundrum. Also your quest-giver would have to accept stolen items :ninja:
It goes on to say that the onions appearing in vendor's stocks are random. So deciding on the number to require depends on how much of a Sadist you are and how much of a Masochist you think your player may be :drool:
I think 15 would be about right.
As for me; I usually put those quests on hold while I do another one and look for the items (onions in this case) at the same time. If I finish the other quest then its
Spoiler player.additem 3368A (for onions)
and on to the next quest.