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There's no specific tag limit. Big server hardware should be able to handle hundreds of thousands of tags. Your Modbus devices are more likely to be the bottleneck than Ignition. You can easily poll at 500ms. Going much faster may require some network analysis (latency matters).
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There's no specific device limit. It would be easiest to manage if you used many tags per device and used Modbus TCP concurrency to tweak performance.
You should use Ignition's 2-hour trial mode to mock up what you need.