It says 'wool blend', but how much wool is actually in it?

The term wool blend does not mean it was woven only with wool, but that wool was included together with other fibers. These pants are just like that. The largest share is Tencel 56%, followed by rayon 22%, wool 20%, and span 2%^[1][2].

The four fibers have different roles. Tencel and rayon are regenerated fibers made by dissolving wood pulp and drawing it back into yarn; their smooth surface and draping property create a silhouette that falls along the leg line when used in pants. Wool traps air between its crimped fibers to hold in warmth, and even at just 2%, span eases the pulling feeling when sitting or walking.

The fact that care instructions are narrowed down to dry cleaning only also connects to this composition. Washing in water, bleaching, and tumble drying must all be avoided, and when drying, it should not be hung, but laid flat^[3]. In general, rayon-based fibers lose strength when wet, easily twisting or shrinking, and when wool combines water, heat, and friction, the fabric felts, making it difficult to restore.

hmmm, the fact that the care label was left as a single line is closer to the combined result of the circumstances where all four fibers are weak against water in their own different ways, rather than just because of the wool 20%.