They could raise application prices…not a perfect solution, but that’s often a tactic for companies with products where demand is great.
They can move to a less holistic admission process, and admit based on stats…or have a first cut based on stats.
They can hire more external readers. Many schools already hire hundreds of readers each year, and pay them around $20 per hour. At that low hourly rate, the app fee covers that cost for many colleges, although there are training costs too.
Fundamentally schools can’t control how many apps they get.