I ran a website and operations for a company that did over 1mm a year in revenue. Everything was done off of a single machine that was the webserver, database

I ran a website and operations for a company that did over 1mm a year in revenue. Everything was done off of a single machine that was the webserver, database server and media server. Had 10k customers all of whom were active daily. Postgres and Python/Django and I never saw load averages much over 10%. Unless you have a substantial fraction of a million daily users you probably don't need mongo.