So if you are only allowed to have 10 infantry models assault, and only 10 infantry models defend, do they have the step up rule?

I'm imagining a scenario where let's say 20 Dryads assault a building held by 20 Saurus. Each side is allowed to designate 10 models to fight in the assault/defense. The Dryads attack first (Initiative 6). Say they kill 3 Saurus. When the Saurus attack on their Initiative, do they only have 7 models that can attack back? Or can they step up 3 models to attack back with 10 models?

I could not find the answer in the BRB, but I may have missed it.