Managing forty customers is a different job from managing forty servers. These guides are about separation, standardisation and reporting: how to roll out one way of working across all customers and still deliver evidence per customer.
PagerDuty starts at 21 dollars per user per month and sends your alert data to the US. For a team of two to ten people it can be done with a self-hosted ntfy server, a rotation schedule in a text file and a fifty-line script: alerts go to whoever is on duty this week, escalate after ten minutes without acknowledgement to the backup, and break through the phone's silent mode. Including one-tap acknowledge.
One Zabbix per customer doesn't scale; one Zabbix for everyone leaks. This is the architecture that does work with open-source building blocks — Prometheus per customer, Grafana organisations, Alertmanager routing on a tenant label, one naming scheme and one offboarding procedure — and the honest bill for what it costs in maintenance at ten, forty and a hundred customers.