Nah, not 100, that's too convenient a number. Make it something weird like 97 or 143 or whatever non-intuitive option. Have someone hint about 90% in that you'll want "all" the rings, but prior to that final quest, nobody tells you how many there are, so you'll be at 97 searching for the last three and going nuts.

Of course, the achievement isn't for finding, but for delivering all of them and only triggers when you're handing them over, otherwise you might know ahead of time that you've got them all.