Microsoft Knowledge Base Article 310072 contains the following summary:

This article describes the support for a large number of logical unit numbers (LUNs) in Windows Server products. When you configure a server with more than eight LUNs, the hardware vendor must be involved in the planning and configuration. There may be several different ways to achieve the configuration you want; the hardware vendor is best equipped to supply the necessary information. This article is not meant to be all-inclusive because of the various implementations that a hardware vendor can use. Contact your hardware manufacturer to determine if and how your hardware can support more than eight LUNs. Windows Server products support up to:

8 buses per adapter
128 target IDs per bus
254 LUNs per target ID