I don't think having a fixed number is a good idea. The number should change as the number of voters in the Assembly changes, to avoid it becoming so low that it's meaningless or so high that it's impossible.

I agree that getting a number of legislators equal to or exceeding one-third of the number of votes in the most recent Assembly vote would be somewhat difficult. I don't think it would be impossible, by any stretch. It would ensure that these resolutions only go to vote when there is at least a significant minority that objects to the Chair's ruling, and not just one or two people.