Gift limit

I believe it is 2 3 + X/3 - Y.
Where X is the number of your giveaways that are marked as received.
Y is the number of giveaways you have deleted.
I could be wrong though. Someone confirm.
Edit: Based on Delta's comment.

You start with three slots. For every three giveaways that are completed successfully, you get another slot. Deleting a giveaway means the loss of a slot.

After 600 giveaways, you will have 200 slots. Just keep working at it. :P

Will this minus point u get by deleting a GA eventually canceled? Once I startet a GA accidentally twice and deleted one of them immediately.
It´s not important for the number of my slots but it´s a lil flaw ;)

http://www.steamgifts.com/discussion/91wob/faqrules-and-guidelines

Understanding Feedback

The feedback system is setup to ensure users maintain a minimum of roughly 75% received on their gifts sent. Three is added to this number to give new users room to get started. For example...

 1.  A user has 35 received, 10 not received (78%). They would be limited to a total of 12 not received before dropping below 75%, plus the 3 given to all users, so 15 in total. Since they already have 10 not received, they could post 5 additional gifts.
 2.  Another user has 10 received, 0 not received (100%). They're able to post 3 not received gifts before reaching 75%. Once again, add the 3 to this number, and they can list 6 additional gifts.