Also, from our experience as a NAV partner, we have more than a few customers with 100+ users (I have heard through the grapevine of a NAV account with 500+ users, but it's not ours). And as for transactions processed a day, we have seen customers who deal with e-Commerce and EDI orders take in thousands of transactions a day, as well as customers with transactions that have 50+ line items per order.
It should be mentioned that with NAV on-prem (or hosted in the cloud), we as a partner have the ability to work with such large customers to scale up resources available to the NAV server (or split up access to NAV service tiers, or cluster the servers to balance the load and ensure good performance).
On Business Central, partners do not have control over this, but I imagine that Microsoft tracks this closely to ensure high availability. I had a conversation with Michael Hammond, Senior Engineering Lead at Microsoft, at a user group conference in May on this subject, and he assured me that these considerations aren't being taken lightly. How they are addressed (how are additional resources made available to a tenant? Is a resource-hungry tenant moved to another server? What is the threshold for this to happen) is beyond my scope of knowledge.
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