Is the league Commish Approval or League Vote? If it's league vote, the people will hold their own. If its Commish Approval and enough people send him requests to veto, I'm sure he'd veto it. As far as how many voted, idk.
I am in a free Yahoo custom league with the "league votes" option. It is a 12 team league. According to the rules, one third of the teams is the number used to veto a trade.
In a 12 team league that means 4 votes against your trade and it's automatically vetoed without any discussion or commissioner intervention.
The Commissioner of this league offered me Philly team defense and Jed Weaver for Drew Bledsoe and Erron Kinney. I accepted but almost 4 hours later the trade was vetoed when somebody claimed to accidentally vote against it. He was confused as to why his mistake triggered a league veto and this caused a controversy in the league. (Nobody voiced an opinion on the trade in the league forum.)
I would prefer to have the commissioner have the last say with vetos rather than have no idea who is quietly vetoing my trade and see it disappear like voodoo magic.