^The Weyoun who first appears in "To the Death" and is killed in the same episode was Weyoun #4.
Weyoun #5 first appears in "Ties of Blood and Water". He is killed off screen in a "transporter accident" in "Treachery, Faith and the Great River" (it is strongly hinted that Damar had him killed).
Weyoun #6 was a defective clone. He first appears and also kills himself in "Treachery, Faith and the Great River".
Weyoun #7 first appears in the same episode and has his neck snapped by Worf after making a disparaging comment about Ezri in "Strange Bedfellows".
Weyoun #8 makes his first appearance in that episode. He is killed, finally, in "What You Leave Behind". Damar's rebels had already destroyed the Dominion cloning facility at that point, meaning there was no way to create another clone.