There's no statutory maximum per staff member, but the WKR sets a free allowance of 1.7% of the taxable payroll up to €400,000 (and 1.18% above that). Exceed it and you pay 80% final levy on the excess.
With a €1 million payroll the free allowance is €16,550 — spread over all WKR provisions combined. Ask your tax adviser for the exact calculation for your situation.