To match tasks/projects that have all matching labels, use an advanced smart list filter:
@label1 @label2 &&
@label1 @label2 && @label3 &&
@label1 @label2 && @label3 && @label4 &&
To match tasks/projects that have any matching labels, use an advance smart list filter:
@label1 @label2 ||
@label1 @label2 || @label3 ||
@label1 @label2 || @label3 || @label4 ||
To match tasks/projects inside any of a number of categories, use an advanced smart list filter:
#parent1 #parent2 ||
#parent1 #parent2 || #parent3 ||
#parent1 #parent2 || #parent3 || #parent4 ||