There are six major candidates remaining in the race to be the Republican nominee for President (I'm going by 

FiveThirtyEight's definition of major candidates

When will the next one drop out, leaving us with just five?

This is linked to the market on who the next dropout will be, so you can see more about the rules about what counts as dropping out, etc. here:

The multiple outcomes on this market aren't linked and the dates are inclusive, so if someone drops out on 14th January, the two December markets will resolve to NO while the 14th January market and 

 of the later markets will resolve to YES.