Forgot to mention - 250 concurrent connections is the limit for concurrent clients as you mentioned. This is within the Apache config file, and in theory, can be edited, though I don't recomend it.
See KB:
http://www.symantec.com/business/support/index?page=content&id=TECH149252
As apache handles this, I presume that once it hits 250 CONCURRENT connections, your clients will get an apache error message. You client will then retry to connect later, and download any updates once the number of connections drops under 250.
I would suggest you contact your account manager for a formal document on this, or one of the Symantec authorised partners for help with the design.