You should be fine.

System -> Service Parameters. Select your server. Cisco TFTP (Active)

Click the advanced button.

Maximum serving count determines how many simultaneous TFTP requests can happen. Click the link to read the official description.

So go through your calculations and set the number appropriately. Probably based on your slowest speed WAN link to the remote site that has the most number of phones.

You can put it back to default when you are done.

I would do it on a downtime window (you probably are anyways) and consider if backups, etc are going on over the WAN.

I don't think there is any determent to the phones waiting for their firmware. Probably eventually a timeout that may try again later (and not register) or revert back to the firmware they currently have (and try again the next time the phone is reset.)

I think it would be pretty cool to setup some kind of distributed firmware server at every remote site so the firmware file goes over the WAN once to update all phones.

The question becomes is there a way to determine what phones have what firmware to see if any were missed?

Actually, I found it. Third party though unless you have programmers on staff. Just go through all phone subnets, connect to the web server and scrape the firmware field from the HTTP. Never done it.

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=IP%20Telephony&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1ddec9f3/0#selected_message