Here is a background script you can use to find all the task types that are used in your ServiceNow Instance. You can use this script in scripts - background or alter slightly to use a script include for a report.
Background Script
findTaskTypesUsed(); function findTaskTypesUsed() { var count = new GlideAggregate('task'); count.addAggregate('COUNT', 'sys_class_name'); count.query(); while (count.next()) { var taskType = count.sys_class_name; var taskTypeCount = count.getAggregate('COUNT', 'sys_class_name'); gs.log("The are currently " + taskTypeCount + " tasks with a task type of " + taskType); } }