Hi @Your Majesty, yes BuildShip is highly scalable and can handle 500k users on your app. It will directly depend on the number of executions processing hours of your BuildShip workflows. The pro plan will give you ~250k executions which will be less to handle your usecase. The Expert plan will be able to handle the requests of 500k user requests as it comes with ~900k executions hours.
You can start with Experts plan initially and as your app scales you can also switch to BYOC (Bring your own cloud -) offering which will allow you to directly to your cloud project and scale at your cloud compute level, with unlimited execution hours upto 5 projects.