I agree with DianaG that exchanging numbers of partners is not an effective way to establish trust or monogomy or STD-less-ness. If I feel that someone has asked me that question in an effort to decide whether I’m worth dating/screwing, I decline to answer (and am usually less interested in dating/screwing him).
But, in my last relationship numbers did come up in the course of us talking openly about sex – things we’d each tried and liked, tried and not liked, not tried but wanted to someday, etc. I was much more experienced than him, and trading numbers was the starting point for that conversation. So I would never expect numbers to come up when I’m still deciding whether to sleep with someone, but once a relationship is established I don’t mind talking about it.
1. How high does the number of previous partners have to get before you are taken aback?
Probably in the several-hundreds, depending on the age of the guy in question.
2. How high does the number have to get before you call things off?
No such thing.
3. What experiences might a person have that would be a deal-breaker?
No such thing.
4. What makes the answer to #s 2 & 3 a deal-breaker for you?
N/A
5. If there is no deal-breaker for you, why not?
Why would there be?
6. Have you ever been in the reverse situation–that is, had a partner be taken aback or break things off because you had too much experience?
At most I’d say that one or two guys have seemed kind of intimidated, but just at first. There’s absolutely no reason for someone with little experience to feel intimidated by someone with a lot more experience, and I always make that very clear. When I’m with someone, the only experience that matters is what I’ve done with him – if he doesn’t feel the same way, we’re probably not a good match.
If anyone has been taken aback or broken up with me because of my history, I’m not aware of it.
[QUOTE=alice_in_wonderland]
Now that’s a real shame. Once you’ve done something 1,000 times, you really should be getting pretty good at it, no?
[/QUOTE]
Not if it’s 1,000 separate women. 
In fact, the lack of skillz could be exactly why the number is so high: it might be easy to get a woman to sleep with you once, but if you’re not that good she probably won’t sleep with you again. I’m almost never interested in the hottie at the bar who has all of the women hanging around him: good looks != good in bed, and I have a pet theory that the best-looking guy in the bar will be the worst one in the sack.*
*The corollary, however, is not true: the ugliest guy in the bar will not be the best one in bed, because you do have to be able to get a woman to sleep with you at least that one time… 