In Treejack and Chalkmark, you may have more tasks than you want to present to your participants. You can opt to have a randomly selected grouping of your tasks
On the survey setup page, at the bottom of the "Tasks" tab, is a group of controls. Simply change the drop-down menu from "all" to the desired value. Note, however, that you must have chosen to randomly order your tasks for each participant in order to select this option.
It's often advisable to show a subset of your tasks if you have a large number of them in order to prevent your participants getting bored and abandoning your survey - we recommend showing each participant no more than 20 tasks. This means, however, that you'll potentially need to invite more participants in order to get enough results for each task.