The slider can hold up to 6 comps.
On the backend, if you click on 'raffles', you will see some stars next to each competition or mark as featured when setting up, it's easier to do in list view. This is how you'd choose which comps you want on the slider.
You can click the stars as many as you want, but it will only choose the 6 you have chosen first.
You can change slider order by going to Marketing and Slider Order to adjust what order they are in.