A: No, you can even use tier 1 for multiple websites.
Tiers 2 and 3 are agency plans.
With tier 2, you can create up to 15 client accounts (separate accounts), but you can't connect your own domain, so your clients will need to use anychat.one to manage their settings, inbox, etc.
With tier 3, you can create an unlimited number of client accounts and connect your own domain with CNAME, allowing your clients to use your domain instead of anychat.one. Additionally, with tier 3, you can create your own plans and charge your customers (you must have a Stripe account).