Certainly no more than three tiers. "Traditional" three-tier, where you have a web server talking to an application server talking to a database server, seems

Certainly no more than three tiers. "Traditional" three-tier, where you have a web server talking to an application server talking to a database server, seems like overkill; I'd get rid of the separate application tier. If your tiers are browser, web API server, database: then three tiers still makes sense.