Minimum
2GB RAM for a small API plus database; 4GB RAM if the database is important.
An early SaaS can run on one well-managed VPS, but it should not run on wishful thinking. Start with enough RAM for the database, workers, deploys, monitoring, and backups.
2GB RAM for a small API plus database; 4GB RAM if the database is important.
2 vCPU / 4GB RAM / 60GB+ SSD for a small but serious production backend.
Upgrade when database memory, background jobs, search, or traffic spikes become visible bottlenecks.
The server needs to handle the API, database, workers, deploys, logs, backups, and monitoring. The early advantage is simplicity. The risk is putting every failure mode on one box.
Use 4GB RAM for a customer-facing SaaS backend if the database lives on the same VPS. Use 2GB only for prototypes or very small internal tools.