VizAlerts does not require recipients to be Tableau Server licensed users. You’ll need to use VizAlerts’ Advanced Alerts feature for that.
As for volumes the functional limits are based on the complexity of each alert, the performance of your Tableau Server, and the number of emails. For example if you’re sending 1500 emails and each email contains a custom per-user viz that takes 30 seconds to render then that alert will take 1500 visualizations * 30 seconds each / 3600 seconds per hour = roughly 12.5 hours (before any multi-threading takes place). So depending on what you’re doing you’ll need to look into configuring multi-threading, your viz performance, etc. That said, I do know of a VizAlerts user that sends out individually customized dashboards to 2,000+ store managers every morning.