Can a MERN SaaS scale to thousands of tenants?

Yes. We have shipped MERN SaaS platforms serving 40,000+ active users across thousands of tenants on a shared-database architecture. The standard scaling toolkit: proper MongoDB indexing on the tenant field across all collections, Redis caching for hot tenant data, BullMQ workers for background processing that would otherwise block the Node.js event loop, read replicas on Atlas when read query load grows, and Node.js cluster mode for horizontal CPU scaling. Tenants with large data volumes or strict compliance requirements get moved to isolated Atlas clusters as their needs justify it.