The number of shown Tasks is set by a Parameter in the Dialog Tasklist in JsfWfUi:
IQueryResult queryResult = ivy.session.findWorkTasks(null, PropertyOrder.create(TaskProperty.ID, OrderDirection.DESCENDING),
0, 25, true, EnumSet.of(TaskState.SUSPENDED, TaskState.RESUMED, TaskState.PARKED));
Replace 25 by -1