I am not familiar with any volume licensing that might be available. You will probably need to consult with a reseller or check out the information for the Partner program. - http://partner.microsoft.com
Note that the current version of OneCare can only be installed on up to 3 PCs with each subscription. Therefore, you would need to set up 8 to 9 subscriptions, each with a unique LiveID. If these computers are part of a single business, that would be rather a pain to manage. The forthcoming Small Business Server will include OneCare for Server and will provide protection for the attached clients, based on the licensing for SBS, but that is currently in beta.