How many times did you have to re-roll and why?

Only twice.
One I cancelled myself after ~10 days of no respond and delivered (even though the winner had majority of a bundle), second got rejected by "support".
It's been on SGv1.

Only one so far - user already had the game he won...

reroll twice for 2 GA
and once for 1 GA

Once. I was the winner in a GA of some minor title. When won i researched the game and found that i've gotten that game in a desura bundle and just forgot about it. Told the gifter, asked for a reroll. Retroactively feels like a mistake now knowing that desura goes down the drain.

I wanted a reroll several times but I actually had only one reroll once and this was because a winner from my whitelist didn't sync his account after winning and won my second giveaway for the game as well.

18 requests.
8 requests for non-activation of previous wins (only started checking these in the last month or two)
6 for the winner letting me know they'd bought or won the game already
3 for the same giveaway of Dragon Age Origins Awakening as they didn't own the base game
1 for the winner still not claiming his gift after I sent to his email, sent through Steam too and even asked him in chat why he didn't claim. He ignored all communications. Think he's actually still on my friends list, lol.

2 non activation ones got denied. First one because they'd already served a suspension, second because the game they hadn't activated isn't even released yet and in a brain fart I thought it was.

Once (in 36 giveaways so far). The winner was suspended, so couldn't redeem the key.

3 times:

• two of my too-successfully-hidden XCOM:EW giveaways were won by the same guy, I got the one with more entries rerolled
• once I won a Critter Chronicles bonus content pack, and it turned out I had it already. I felt a bit embarassed, but the GA creator assured me that I was actually the 2nd winner sending back the same gift with this reason - so at least I was not alone
• Mass Effect 2 was initially won by someone who just entered to ensure there were 5 entries (at the end there were 8, but the last 3 entries popped up only at the very end)

Once, because the winner requested it.

17 approved rerolls, so a little bit less than 10% of my GA. Reasons: special rule break (previous version of SG where custom rules were allowed), blacklisted users (previous version too, where blacklists were manual), recent not activated win(s) / regifting, currently suspended winner...

None. Eventually I managed to contact all of the unresponsive ones.

5
"Previously Won the Game, or Already on Account"
"Winner Rejected Gift"
"Previously Won the Game, or Already on Account"
"Did Not Meet Requirements"
"Did Not Meet Requirements"