APEX does not span instances by default, so a "normal" installation would be 1 APEX per database instance. I've seen customers use database links and/or REST services to let an APEX instance access other databases, but it sounds to me like these 67 government would be expecting some isolation between them, ie, they are standalone entities.

However, given you have 4 servers have you given some thought to perhaps running pluggable databases instead of full blown instances? Even if you didn't want to cough up a multitenant license, you can have up to 3 free pluggables, which might get from 67 instances to (say) 25.

Pluggable then gives you some more APEX options because it can be at the CDB level or each pluggable.

When it comes to ORDS, you have plenty of flexibility because a single ORDS can manage multiple databases. So assuming you have "n" application servers, a common setup would an ORDS instance per server, each handling multiple databases. See chapter 3 "Configuring Oracle REST Data Services for Multiple Databases" in https://docs.oracle.com/en/database/oracle/oracle-rest-data-services/25.1/ordig/oracle-rest-data-services-installation-and-configuration-guide.pdf

Hoever, some times its business demands that drive the ORDS architecture. If ORDS is serving more then 1 municipality, then things like patching/upgrade/etc require coordination between them. Only you can know if that is workable. If each municipality expects total independence, then they'd naturally expect their own ORDS instance