The NDI / SRT input limit on the Capture 2 Cloud is up to 4 channels per instance, but it’s more of a question of the CPU/Mem on the instance and not the software limitation.
The recommended AWS EC2 instance type depends on which codec you are using for primary and for proxy. We usually recommend that you do 1 or 2 channels per instance but isolation is the best way in that case for avoiding potential issues on a large number of channels. If you need that sort of flexibility - it may be best to run just 1 channel per instance and scale the instance count accordingly to your needs. This can be done with autoscaling groups in AWS for example, or any other tools for infrastructure set up.