Perspective client requirements are pretty minimal. Any device with a modern web browser (HTML 5/CSS 3) will run it. This includes mobile (IOS/Android) and thin clients.
For servers, more is better and “it depends” on usage. In general, 16GB RAM and 4+ modern CPU cores (8+ threads), especially “server” grade, will provide significant capability. Going to 8 core and 32GB would be a good bet for the future. “Bare metal” servers are best, but VMs can be configured for performance (e.g. VMWare “latency sensitivity” setting to high). At a point not far beyond that, large CPU and RAM investments will not add much - other bottlenecks become significant (disk, I/O, etc). At that point a “scale-out” architecture delivers. This happens at a pretty modest point from the perspective of modern server hardware.
For “I/O” functionality, such as: tag hosting (up to >100k tags), database connectivity, alarming, SFCs, reporting, historian. 4 modern CPU cores/8 threads, 16 GB RAM. More memory is better for many Ignition tags. More CPU capability for large historian and reporting capability. When “scaling out” these are “stateful”, so use Redundant Ignition servers and just keep adding. Anecdotally, I’ve heard of a customer using 12M tags with 24 I/O servers (Ignition gateways). At scale configuration becomes significant.
“Frontend” Ignition Gateways. Responsible for Perspective/Vision sessions. 8GB+ RAM (recommend 16+), 4 modern CPU cores (8+ threads). More CPU is better. These are “stateless”, so do not use Ignition Redundancy with “scaling out” architecture. If scaling (past ~100-200 concurrent clients), recommend using a load balancing scheme with “Sticky Sessions”. The workload will be split and gateways can come up or down with minimal client disruption. This scheme works well with cloud technologies.
SQL Database - Similar specs to I/O server. First step should be separating from Ignition gateway. Solid state storage is a must when writing rapidly (historizing large number of tag changes/sec). It’s possible to archive older data to slower storage. Leverage IT - configuration, maintenance, updates, security, backup, administration, standards will likely be more significant issues than hardware specs.
As Phil recommended, try out your setup with a free Ignition trial. I recommend consulting with IA or an experienced Integrator when getting into large architectures.