The basic process is to add up the power requirements of the devices that will be on the circuit and then size the circuit appropriately. Typically you don't want to exceed 80% of the circuit's capacity, which for a 20 amp circuit would be 16 amps or 1920 watts (watts = amps x voltage) because I think this would be considered a "continuous load" because these devices would be on 24/7. Just very quickly looking through Cisco's site I didn't see anything about the power requirements but I'm sure it's there.

The only real difference in running a 20 amp circuit vs. a 15 amp is the cost of the wire. Around here 12/2 NM-B (aka Romex) is about 60% more expensive than 14/2. Assuming a 50 ft. run, that would be about $11.

Routers and hubs are pretty low power consumers these days (this is a far cry from 25 years ago when the initial Cisco routers we had used a few amps just to run the giant blower they had that kept the thing from melting down). 15A probably would suffice, but as Mike alludes, the cabling price difference is going to be minimal in the scope of things. I'd run 12-2 and put in a 20A circuit.

I think from a power standpoint, you're talking about 2-4 amps for each unit, and maybe a bit less depending on load.

I think, since you're talking about a closet, your main issue will be ventilation. You'll need to keep the temperature below 90F, preferably around 75F.

Yes, I agree. The ventilation is def. going to be the main concern with an enclosed area, but I have worked with TC closets before, so no real advise needed in that category. The power I'm just not sure about because I've never done it. So General consensus is my original plan of 20A breaker on 12 gauge wire?