Where can I find the documentation for a Cisco routers capabilities?

for example - I have a router that is the hub of an encrypted DMVPN (phase 1) and I want to know

• maximum dmvpn spokes
• maximum encrypted throughput

Here is the product page https://www.cisco.com/c/en/us/products/routers/3925-integrated-services-router-isr/index.html

https://www.cisco.com/c/en/us/support/routers/3925-integrated-services-router-isr/model.html

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I guess this leads me into another question -- when you are given requirements how do you go about selecting the gear you need?