That depends on what you mean by minimal. Every (sanely managed) carrier has spares. They could park 24 Maxs tomorrow and cover those routes with spares from among the mainline fleet of ~940 non-MAX aircraft. They could cut scheduled flights (say, effective 4/1) to build back the available spares to a normal level. AA had to ground 80 MD-80s immediately back in 2008 to inspect wiring bundles. Post-merger the fleet's a lot bigger today.