It depends on which services, but for simpler services with well understood open source equivalents (e.g. route53 and BIND) you can probably manage two per sysadmin at a similar quality to AWS with a basic API. For something like RDS or Redshift, if you're very lucky you'll get away with one sysadmin per service.

This covers not just running the services, but keeping up to date with security updates, scaling the systems, feature additions, bug fixes and keeping up with the state of the art.

Services like RDS are an amazing net cost saving for small to medium users. Something like Cloudfront would cost you a million just to get going. Raw EC2 is nearer break even.