My winner hasn't ticked the box on his end

It's like virtually unlimited if you're a developer.
FAQ

Developers

Q. I'm a game developer or publisher looking to promote my game through giveaways. How do I get started?

A.If you're looking to promote a game, the best option is to create a featured giveaway. These giveaways are displayed at the top of our homepage, and receive the highest visibility. Featured giveaways are created in the same manner as every other giveaway, and our site will automatically feature giveaways on our homepage if they contain 50 copies of a game or greater, and if they're ending within four days. We feature up to two giveaways at once, and if more are available, then two will be displayed at random, with preference given to giveaways of higher copies. Please note, the giveaway will only be displayed to users that have permission to join, and if you choose to limit your giveaway to a particular Steam group, it will only be visible on the homepage to members of that group. If the limitations on your account do not allow you to create a giveaway for 50 copies or greater, then please contact support. We'll upgrade your account to developer status, allowing you to create giveaways up to 100,000 copies.

Creating

Q. Why is there a limit to the number of gifts I'm able to give away?

A.We limit the number of gifts a user can create for security purposes, and to prevent a new user from potentially posting a large number of invalid gifts to the site. When you create giveaways and your gifts are received, this number will increase. At that time you'll be able to create giveaways with a greater number of copies, and more simultaneous giveaways.

If this is not the case, you will need to increase the number of correct deliveries.

so why complaining if you already know?

It ended 3 hours ago and you're already complaining?
You do know people have lives apart from SG?

When you just started on the website, you have a limited number of GAs you can make. IIRC, when your GAs end and the product is delivered, that limit will rise and rise. If you have a good historic of GAs as well, you can contact support and ask for more limit, but that wouldn't work for you, since you just started.

So yeah, in your case, just sit tight and wait :) Welcome to SG!

I was expecting that game to have ended 3 days ago, not 3 hours ago...

You joined SG 7 hours ago. There's a high chance you'll make a wrong giveaway or misunderstand local rules, so just wait a bit, get some experience. Poke around, read some useful threads, join the active community of SG, just have fun.