The only Desolate Council member that can be positively confirmed as defecting was Parqual Fintallas, who wished to remain with his Human daughter and felt that Sylvanas was unlikely to allow of the two of them to continue any form of relationship after the Gathering (and who had apparently had inclinations in this direction before the Gathering had even commenced). Even the leader of the Desolate Council, Elsie/Vellcinda, was completely loyal to Sylvanas and told Calia that even though she was the true heir of Lordaeron and last of the Menethil line she considered Sylvanas to be her queen. The rest of the Council's loyalties will never be known, as they were executed wherever they stood. The time between Sylvanas' horn signaling the end of the dispatching of the Dark Rangers was very short, and only those Council members who already returned to the Forsaken base-camp (well before the call to retreat) were "forgiven" their trespass. Nathanos said many of them were returning to the Forsaken pitch at the time of the executions, and others were caught mid-meeting and unsure of where to go or what to do. Parqual and his daughter were booking it toward Stromgarde when he was cut down by a lethal arrow.
Sylvanas did kill her own people, and some of them (especially Elsie/Vellcinda) were completely innocent any charge of defection. The number doesn't really matter at the end of the day - it is the symbol of those deaths that will likely prevail in the minds of the Alliance and even her own people.