Trying for 8 instead of 6 would reduce your odds only slightly.
For example, if the quota for your date only had 7 open slots left, then your request for 8 slots would fail, but a request for 6 would succeed.
But it is far more likely that with either number, your requested date would be wide open (if your application was processed early in the lottery), or the date would be full.
I would try for the 8 slots. But understand, too, that weekends fill up earlier than weekdays. So put down your ideal dates, but consider trying for a Monday, Tuesday or Wednesday entry date, too, since those are usually less desirable, so would fill more slowly.
