Try a few designs with various combinations of which app pools and which sites on which servers. And determine a safe capacity level for your performance objectives. There is not one correct answer when doing capacity planning.
A few years ago this very Stack Overflow network covered their architecture and noted that the entire network of Q&A sites can run off of one application on one server. In reality, just the web app tier runs on like 9 servers because hardware is inexpensive, enabling performance and high availability as features. Stack Exchange is probably an outlier by a few metrics, but its definitely simple to dedicate a few hosts to varies services in the architecture, and not worry too much about RAM and CPU.
Also consider whether you want to split up the resources into many virtual machines or containers. Allowing operating system maintenance to affect less of the total capacity at once. Possibly getting better resource utilization. At the price of having more operating system instances and a hypervisor layer to manage.