I went for 20k because I'm a pessimist, but I'm hoping for as many backers as possible.
It's interesting to note that Playtonic's Facebook has 9,319 likes, whilst A Hat In Time has 9,024 likes. They're pretty similar, but one only opened a couple of months ago and the other has been open since August 2012. It's a good sign that Project Ukulele will get funded more than A Hat In Time, I think...
But it's really down to the zeitgeist of the internet. Kickstarter was 'the thing' a couple of years ago. Everyone was backing stuff. Even a $55,000 potato salad party was funded. Are people still in the mood to fund things? I hope so.
I think media coverage, awareness and the general attitude of the population will contribute to how many people decide to donate.
Edit: I also want to say, when people claim that because someone has, for example, 1.7 million subscribers, that doesn't mean all of them are going to put money towards something. Half of them might not even use YouTube any more, another quarter might be kids without a bank balance etc.