How is the number of GA available to create determined?

Three for new members plus one for each three giveaways marked as received.

You start with 3 giveaway slots. For every 3 giveaways you successfully complete and get marked as received, you get an additional slot.

When you delete a giveaway or don't get a giveaway marked as received, the slot involved is lost. If you ever drop down to 0 slots (say, by messing up and having to delete all your giveaways), you can contact support to ask them to reset you back to 3 slots. If you're planning on giving away a large number of giveaways at once, sometimes staff will also be willing to increase your available slots.

Thanks for clarification guys :)