Following are the excerpts from the above Sizing Guide.
Although there is no hard limit on the number of client devices that can be deployed below MX Security Appliances, for purposes of this document all tests were performed with the client counts shown in the table below. Exceeding these client counts may result in performance that varies from the sizing data contained in this guide.
UTM products come with a variety of security and networking features. Understanding the benefits and tradeoffs of these features is crucial to getting the maximum security benefit without unnecessary performance degradation.
One thing that is worth mentioning is that a single client will have multiple connections passing through the firewall so the more client devices you have the number of connections increases very quickly. Any app or service on your device neds its own connection.
For example my computer right now have 38 active connections going through my firewall. Each connection requires resources to inspect the traffic and apply any rules whether they are access or QOS.
Well done for reaching out since you weren't sure, the last thing you want to do is underspec and end up having to replace the unit.